[Tex/LaTex] Making arrow with a gradual colour

colortikz-pgf

I want to make an arrow that has gradual color as shown in the picture below:

enter image description here

First I wanted to make that arrow in one stroke, but I do not know how to make shaded color in postaction, so in the end I draw right side first, left side second and put shade over it in the middle.

Now two problems appear.

  1. Color shading is not perfect, the middle colors do not correspond well to colors on the left and on the right. I found out that the problem comes from the fact I opened xcolor in cmyk mode.
  2. The thickness of the colored part of the arrow is not perfectly constant. This cannot be seen on the picture but on the MWE-created PDF.

Is there any other way to create such effect, let's say in one stroke? If not, how can colors created by \draw be aligned with colors created by \shade?

IMPORTANT EDIT: I would prefer a tweak to the existing solution, if possible. Namely I am already using dozens of such arrows without color gradient going between two specified points following a bent path, i.e. (0,0) — (0.5,0) arc (-90:0:0.5) — (1,1). Now I just need ONE straight but with gradient color.

MWE: 

\documentclass{minimal}

\usepackage[cmyk]{xcolor}
\usepackage{tikz}
\usetikzlibrary{arrows.meta}

\begin{document}

\begin{tikzpicture}

\draw[line width=4.15mm,{Triangle Cap[reversed,cap angle=120]}-,postaction={draw,color=red!75,line width=3.85mm,shorten <=0.25mm}] (0,0) -- (-0.8,0);
\draw[line width=4.15mm,-{Triangle Cap[cap angle=120]},postaction={draw,color=red!25,line width=3.85mm,shorten >=0.21mm}] (-0.8,0) -- (-1.6,0);
\shade[left color=red!25,right color=red!75] (-0.6,-0.5*3.85mm) rectangle (-1.0,0.5*3.85mm);

\end{tikzpicture}
\end{document}

Best Answer

Explanation

Relating to the question of this comment:

I am impressed by the amount of effort put into the solution. As you understand both solutions, what would you recommend me to use, overlapping or postaction solution? – Pygmalion

With the first approach you can also use postaction, but shading path trims the arrow, concretely, in the case of the path (0,0) -- (0.5,0) arc (-90:0:0.5) -- (1,1) (not in the others):

enter image description here

enter image description here

\begin{tikzpicture}
\path[draw,line width=4.15mm,{Triangle Cap[reversed,cap angle=120,bend]}-{Triangle Cap[cap angle=120,bend]},postaction={shading=shading1,shading angle=45,shading path={draw=transparent!0,{Triangle Cap[reversed,cap angle=120,bend]}-{Triangle Cap[cap angle=120,bend]},line width=3.85mm,shorten <=0.25mm,shorten >=0.21mm}}] (0,0) -- (0.5,0) arc (0:90:0.5) -- (1,1);
\end{tikzpicture}
\begin{tikzpicture}
\path[draw,line width=4.15mm,{Triangle Cap[reversed,cap angle=120]}-{Triangle Cap[cap angle=120]},postaction={shading=shading1,shading path={draw=transparent!0,{Triangle Cap[reversed,cap angle=120]}-{Triangle Cap[cap angle=120]},line width=3.85mm,shorten <=0.25mm,shorten >=0.21mm}}] (0,0) -- (1.6,0);
\end{tikzpicture}
\begin{tikzpicture}
\path[draw,line width=4.15mm,{Triangle Cap[reversed,cap angle=120,bend]}-{Triangle Cap[cap angle=120,bend]},postaction={shading=shading1,shading angle=37,shading path={draw=transparent!0,{Triangle Cap[reversed,cap angle=120,bend]}-{Triangle Cap[cap angle=120,bend]},line width=3.85mm,shorten <=0.25mm,shorten >=0.21mm}}] (0,0) -- ++(15:1) -- +(45:1);
\end{tikzpicture}
\begin{tikzpicture}
\path[draw,line width=4.15mm,{Triangle Cap[reversed,cap angle=120,bend]}-{Triangle Cap[cap angle=120,bend]},postaction={shading=shading1,shading angle=10,shading path={draw=transparent!0,{Triangle Cap[reversed,cap angle=120,bend]}-{Triangle Cap[cap angle=120,bend]},line width=3.85mm,shorten <=0.25mm,shorten >=0.21mm}}] (0,0) arc (180:50:2);
\end{tikzpicture}

Therefore, with an "overlapping" works well, because both arrows are trimmed.

With the second/third approach (third code version) you can pass other options such as shading xsep=<length> and shade path sorts everything else out, see please the answer of Mark Wibrow:

enter image description here

Of course you can also overlap with the second/third approach.

IMHO: For straight arrows the best solution is user191948´s answer. For bend arrows you have two approaches, marmot´s and mine.

Third approach, with improved postaction:

The next improvements are based on the comment of marmot, see below:

If you load the bending library and add bend to all the arrows along curved paths, the result looks much nicer IMHO. Also adjusting the shading angle may improve the appearance. – marmot

Output shading1:

enter image description here

Output shading2:

enter image description here

Code:

\documentclass{article}

\usepackage[cmyk]{xcolor}
\usepackage{tikz}
\usetikzlibrary{fadings,decorations.pathmorphing,arrows.meta,shapes,bending,shadings}

%https://tex.stackexchange.com/questions/137357/how-to-draw-an-arrow-with-two-colors
\makeatletter
\newif\iftikz@shading@path

\tikzset{
    % There are three circumstances in which the fading sep is needed:
    % 1. Arrows which do not update the bounding box (which is most of them).
    % 2. Line caps/joins and mitres that extend outside the natural bounding 
    %    box of the path (these are not calculated by PGF).
    % 3. Other reasons that haven't been anticipated.
    shading xsep/.store in=\tikz@pathshadingxsep,
    shading ysep/.store in=\tikz@pathshadingysep,
    shading sep/.style={shading xsep=#1, shading ysep=#1},
    shading sep=0.0cm,
}

\def\tikz@shadepath#1{% 
    % \tikz@addmode installs the `modes' (e.g., fill, draw, shade) 
    % to be applied to the path. It isn't usualy for doing more
    % changes to the path's construction.
    \iftikz@shading@path%
    \else%
    \tikz@shading@pathtrue%
    % Get the current path.
    \pgfgetpath\tikz@currentshadingpath%
    % Get the shading sep without setting any other keys.
    \begingroup%
    \pgfsys@beginscope% <- may not be necessary
    \tikzset{#1}%
    \xdef\tikz@tmp{\noexpand\def\noexpand\tikz@pathshadingxsep{\tikz@pathshadingxsep}%
        \noexpand\def\noexpand\tikz@pathshadingysep{\tikz@pathshadingysep}}%
    \pgfsys@endscope%
    \endgroup
    \tikz@tmp%
    % Get the boudning box of the current path size including the shading sep
    \pgfextract@process\pgf@shadingpath@southwest{\pgfpointadd{\pgfqpoint{\pgf@pathminx}{\pgf@pathminy}}%
        {\pgfpoint{-\tikz@pathshadingxsep}{-\tikz@pathshadingysep}}}%%
    \pgfextract@process\pgf@shadingpath@northeast{\pgfpointadd{\pgfqpoint{\pgf@pathmaxx}{\pgf@pathmaxy}}%
        {\pgfpoint{\tikz@pathshadingxsep}{\tikz@pathshadingysep}}}%
    % Clear the path
    \pgfsetpath\pgfutil@empty%                          
    % Save the current drawing mode and options.
    \let\tikz@options@saved=\tikz@options%
    \let\tikz@mode@saved=\tikz@mode%
    \let\tikz@options=\pgfutil@empty%
    \let\tikz@mode=\pgfutil@empty%
    % \tikz@options are processed later on.
    \tikz@addoption{%
        \pgfinterruptpath%
        \pgfinterruptpicture%
        \begin{tikzfadingfrompicture}[name=.]
            \pgfscope%
            \tikzset{shade path/.style=}% Make absolutely sure shade path is not inherited.
            \path \pgfextra{%
                % Set the softpath. Any transformations,draw=none} in #1 will have no effect.
                % This will *not* update the bounding box...
                \pgfsetpath\tikz@currentshadingpath%
                % ...so it is done manually.
                \pgf@shadingpath@southwest
                \expandafter\pgf@protocolsizes{\the\pgf@x}{\the\pgf@y}%
                \pgf@shadingpath@northeast%
                \expandafter\pgf@protocolsizes{\the\pgf@x}{\the\pgf@y}%
                % Install the drawing modes and options.
                \let\tikz@options=\tikz@options@saved%
                \let\tikz@mode=\tikz@mode@saved%
            };
            % Now get the bounding box of the picture.
            \xdef\pgf@shadingboundingbox@southwest{\noexpand\pgfqpoint{\the\pgf@picminx}{\the\pgf@picminy}}%
            \xdef\pgf@shadingboundingbox@northeast{\noexpand\pgfqpoint{\the\pgf@picmaxx}{\the\pgf@picmaxy}}%
            \endpgfscope
        \end{tikzfadingfrompicture}%
        \endpgfinterruptpicture%
        \endpgfinterruptpath%
        % Install a rectangle that covers the shaded/faded path picture.
        \pgftransformreset%
        \pgfpathrectanglecorners{\pgf@shadingboundingbox@southwest}{\pgf@shadingboundingbox@northeast}%
        %
        % Reset all modes.
        \let\tikz@path@picture=\pgfutil@empty%
        \tikz@mode@fillfalse%
        \tikz@mode@drawfalse%
        %\tikz@mode@tipsfalse%   <- To have successful compilation with pgf-tikz v3.0.1a
        \tikz@mode@doublefalse%
        \tikz@mode@clipfalse%
        \tikz@mode@boundaryfalse%
        \tikz@mode@fade@pathfalse%
        \tikz@mode@fade@scopefalse%
        % Now install shading options.
        \tikzset{#1}%
        \tikz@mode%
        % Make the fading happen.
        \def\tikz@path@fading{.}%
        \tikz@mode@fade@pathtrue%
        \tikz@fade@adjustfalse%
        % Shift the fading to the mid point of the rectangle
        \pgfpointscale{0.5}{\pgfpointadd{\pgf@shadingboundingbox@southwest}{\pgf@shadingboundingbox@northeast}}%
        \edef\tikz@fade@transform{shift={(\the\pgf@x,\the\pgf@y)}}%
        \pgfsetfading{\tikz@path@fading}{\tikz@do@fade@transform}%
        \tikz@mode@fade@pathfalse%              
    }%
    \fi%
}
\tikzset{
    shade path/.code={%
        \tikz@addmode{\tikz@shadepath{#1}}%
    }
}
\makeatother % <- To close the \makeatletter call
%-------------------------------------------------------------------------------------
%https://tex.stackexchange.com/questions/197793/how-to-draw-gradient-arrows-with-tikz
\makeatletter
\def\createshadingfromlist#1#2#3{%
    \pgfutil@tempcnta=0\relax
    \pgfutil@for\pgf@tmp:={#3}\do{\advance\pgfutil@tempcnta by1}%
    \ifnum\pgfutil@tempcnta=1\relax%
    \edef\pgf@spec{color(0)=(#3);color(100)=(#3)}%
    \else%
    \pgfmathparse{50/(\pgfutil@tempcnta-1)}\let\pgf@step=\pgfmathresult%
    %
    \pgfutil@tempcntb=1\relax%
    \pgfutil@for\pgf@tmp:={#3}\do{%
        \ifnum\pgfutil@tempcntb=1\relax%
        \edef\pgf@spec{color(0)=(\pgf@tmp);color(25)=(\pgf@tmp)}%
        \else%
        \ifnum\pgfutil@tempcntb<\pgfutil@tempcnta\relax%
        \pgfmathparse{25+\pgf@step/4+(\pgfutil@tempcntb-1)*\pgf@step}%
        \edef\pgf@spec{\pgf@spec;color(\pgfmathresult)=(\pgf@tmp)}%
        \else%
        \edef\pgf@spec{\pgf@spec;color(75)=(\pgf@tmp);color(100)=(\pgf@tmp)}%
        \fi%
        \fi%
        \advance\pgfutil@tempcntb by1\relax%
    }%
    \fi%
    \csname pgfdeclare#2shading\endcsname{#1}{100}\pgf@spec%
}
\makeatother

\createshadingfromlist{shading1}{horizontal}{red,white}
\createshadingfromlist{shading2}{horizontal}{red,yellow,green,cyan,blue}

\begin{document}


    \begin{tikzpicture}
    \draw[draw,line width=4.15mm,{Triangle Cap[reversed,cap angle=120,bend]}-{Triangle Cap[cap angle=120,bend]},postaction={draw=white, shade path={shading xsep=0.6cm,shading=shading1,shading angle=45}, line width=3.85mm,{Triangle Cap[reversed,cap angle=120,bend]}-{Triangle Cap[cap angle=120,bend]},shorten <=0.25mm,shorten >=0.21mm}] (0,0) -- (0.5,0) arc (0:90:0.5) -- (1,1);
    \end{tikzpicture}
    \begin{tikzpicture}
    \draw[draw,line width=4.15mm,{Triangle Cap[reversed,cap angle=120]}-{Triangle Cap[cap angle=120]},postaction={draw=white, shade path={shading xsep=-0.5cm, shading=shading1},line width=3.85mm,{Triangle Cap[reversed,cap angle=120]}-{Triangle Cap[cap angle=120]},shorten <=0.25mm,shorten >=0.21mm}] (0,0) -- (1.6,0);
    \end{tikzpicture}
    \begin{tikzpicture}
    \draw[draw,line width=4.15mm,{Triangle Cap[reversed,cap angle=120,bend]}-{Triangle Cap[cap angle=120,bend]},postaction={draw=white, shade path={shading xsep=0cm, shading=shading1,shading angle=37},line width=3.85mm,{Triangle Cap[reversed,cap angle=120,bend]}-{Triangle Cap[cap angle=120,bend]},shorten <=0.25mm,shorten >=0.21mm}] (0,0) -- ++(15:1) -- +(45:1);
    \end{tikzpicture}
    \begin{tikzpicture}
    \draw[draw,line width=4.15mm,{Triangle Cap[reversed,cap angle=120,bend]}-{Triangle Cap[cap angle=120,bend]},postaction={draw=white, shade path={shading xsep=0cm, shading=shading1,shading angle=10},line width=3.85mm,{Triangle Cap[reversed,cap angle=120,bend]}-{Triangle Cap[cap angle=120,bend]},shorten <=0.25mm,shorten >=0.21mm}] (0,0) arc (180:50:2);
    \end{tikzpicture}

\end{document}

Second approach, with postaction:

With the third version of the answer 1 is also possible to achieve the same result with postaction:

Output shading1:

enter image description here

Output shading2:

enter image description here

Code:

\documentclass{article}

\usepackage[cmyk]{xcolor}
\usepackage{tikz}
\usetikzlibrary{fadings,decorations.pathmorphing,arrows.meta,shapes}

%https://tex.stackexchange.com/questions/137357/how-to-draw-an-arrow-with-two-colors
\makeatletter
\newif\iftikz@shading@path

\tikzset{
    % There are three circumstances in which the fading sep is needed:
    % 1. Arrows which do not update the bounding box (which is most of them).
    % 2. Line caps/joins and mitres that extend outside the natural bounding 
    %    box of the path (these are not calculated by PGF).
    % 3. Other reasons that haven't been anticipated.
    shading xsep/.store in=\tikz@pathshadingxsep,
    shading ysep/.store in=\tikz@pathshadingysep,
    shading sep/.style={shading xsep=#1, shading ysep=#1},
    shading sep=0.0cm,
}

\def\tikz@shadepath#1{% 
    % \tikz@addmode installs the `modes' (e.g., fill, draw, shade) 
    % to be applied to the path. It isn't usualy for doing more
    % changes to the path's construction.
    \iftikz@shading@path%
    \else%
        \tikz@shading@pathtrue%
        % Get the current path.
        \pgfgetpath\tikz@currentshadingpath%
        % Get the shading sep without setting any other keys.
        \begingroup%
            \pgfsys@beginscope% <- may not be necessary
            \tikzset{#1}%
            \xdef\tikz@tmp{\noexpand\def\noexpand\tikz@pathshadingxsep{\tikz@pathshadingxsep}%
                \noexpand\def\noexpand\tikz@pathshadingysep{\tikz@pathshadingysep}}%
            \pgfsys@endscope%
        \endgroup
        \tikz@tmp%
        % Get the boudning box of the current path size including the shading sep
        \pgfextract@process\pgf@shadingpath@southwest{\pgfpointadd{\pgfqpoint{\pgf@pathminx}{\pgf@pathminy}}%
            {\pgfpoint{-\tikz@pathshadingxsep}{-\tikz@pathshadingysep}}}%%
        \pgfextract@process\pgf@shadingpath@northeast{\pgfpointadd{\pgfqpoint{\pgf@pathmaxx}{\pgf@pathmaxy}}%
            {\pgfpoint{\tikz@pathshadingxsep}{\tikz@pathshadingysep}}}%
        % Clear the path
        \pgfsetpath\pgfutil@empty%                          
        % Save the current drawing mode and options.
        \let\tikz@options@saved=\tikz@options%
        \let\tikz@mode@saved=\tikz@mode%
        \let\tikz@options=\pgfutil@empty%
        \let\tikz@mode=\pgfutil@empty%
        % \tikz@options are processed later on.
        \tikz@addoption{%
            \pgfinterruptpath%
            \pgfinterruptpicture%
                \begin{tikzfadingfrompicture}[name=.]
                \pgfscope%
                    \tikzset{shade path/.style=}% Make absolutely sure shade path is not inherited.
                    \path \pgfextra{%
                        % Set the softpath. Any transformations,draw=none} in #1 will have no effect.
                        % This will *not* update the bounding box...
                        \pgfsetpath\tikz@currentshadingpath%
                        % ...so it is done manually.
                        \pgf@shadingpath@southwest
                        \expandafter\pgf@protocolsizes{\the\pgf@x}{\the\pgf@y}%
                        \pgf@shadingpath@northeast%
                        \expandafter\pgf@protocolsizes{\the\pgf@x}{\the\pgf@y}%
                        % Install the drawing modes and options.
                        \let\tikz@options=\tikz@options@saved%
                        \let\tikz@mode=\tikz@mode@saved%
                    };
                    % Now get the bounding box of the picture.
                    \xdef\pgf@shadingboundingbox@southwest{\noexpand\pgfqpoint{\the\pgf@picminx}{\the\pgf@picminy}}%
                    \xdef\pgf@shadingboundingbox@northeast{\noexpand\pgfqpoint{\the\pgf@picmaxx}{\the\pgf@picmaxy}}%
                    \endpgfscope
                \end{tikzfadingfrompicture}%
            \endpgfinterruptpicture%
            \endpgfinterruptpath%
            % Install a rectangle that covers the shaded/faded path picture.
            \pgftransformreset%
            \pgfpathrectanglecorners{\pgf@shadingboundingbox@southwest}{\pgf@shadingboundingbox@northeast}%
            %
            % Reset all modes.
            \let\tikz@path@picture=\pgfutil@empty%
            \tikz@mode@fillfalse%
            \tikz@mode@drawfalse%
            %\tikz@mode@tipsfalse%   <- To have successful compilation with pgf-tikz v3.0.1a
            \tikz@mode@doublefalse%
            \tikz@mode@clipfalse%
            \tikz@mode@boundaryfalse%
            \tikz@mode@fade@pathfalse%
            \tikz@mode@fade@scopefalse%
            % Now install shading options.
            \tikzset{#1}%
            \tikz@mode%
            % Make the fading happen.
            \def\tikz@path@fading{.}%
            \tikz@mode@fade@pathtrue%
            \tikz@fade@adjustfalse%
            % Shift the fading to the mid point of the rectangle
            \pgfpointscale{0.5}{\pgfpointadd{\pgf@shadingboundingbox@southwest}{\pgf@shadingboundingbox@northeast}}%
            \edef\tikz@fade@transform{shift={(\the\pgf@x,\the\pgf@y)}}%
            \pgfsetfading{\tikz@path@fading}{\tikz@do@fade@transform}%
            \tikz@mode@fade@pathfalse%              
        }%
    \fi%
}
\tikzset{
    shade path/.code={%
        \tikz@addmode{\tikz@shadepath{#1}}%
    }
}
\makeatother % <- To close the \makeatletter call
%-------------------------------------------------------------------------------------
%https://tex.stackexchange.com/questions/197793/how-to-draw-gradient-arrows-with-tikz
\makeatletter
\def\createshadingfromlist#1#2#3{%
    \pgfutil@tempcnta=0\relax
    \pgfutil@for\pgf@tmp:={#3}\do{\advance\pgfutil@tempcnta by1}%
    \ifnum\pgfutil@tempcnta=1\relax%
    \edef\pgf@spec{color(0)=(#3);color(100)=(#3)}%
    \else%
    \pgfmathparse{50/(\pgfutil@tempcnta-1)}\let\pgf@step=\pgfmathresult%
    %
    \pgfutil@tempcntb=1\relax%
    \pgfutil@for\pgf@tmp:={#3}\do{%
        \ifnum\pgfutil@tempcntb=1\relax%
        \edef\pgf@spec{color(0)=(\pgf@tmp);color(25)=(\pgf@tmp)}%
        \else%
        \ifnum\pgfutil@tempcntb<\pgfutil@tempcnta\relax%
        \pgfmathparse{25+\pgf@step/4+(\pgfutil@tempcntb-1)*\pgf@step}%
        \edef\pgf@spec{\pgf@spec;color(\pgfmathresult)=(\pgf@tmp)}%
        \else%
        \edef\pgf@spec{\pgf@spec;color(75)=(\pgf@tmp);color(100)=(\pgf@tmp)}%
        \fi%
        \fi%
        \advance\pgfutil@tempcntb by1\relax%
    }%
    \fi%
    \csname pgfdeclare#2shading\endcsname{#1}{100}\pgf@spec%
}
\makeatother

\createshadingfromlist{shading1}{vertical}{red,white}
\createshadingfromlist{shading1b}{horizontal}{red,white}
\createshadingfromlist{shading2}{vertical}{red,yellow,green,cyan,blue}
\createshadingfromlist{shading2b}{horizontal}{red,yellow,green,cyan,blue}
\begin{document}


\begin{tikzpicture}
\draw[draw,line width=4.15mm,{Triangle Cap[reversed,cap angle=120]}-{Triangle Cap[cap angle=120]},postaction={draw=white, shade path={shading xsep=0.6cm, shading=shading1}, line width=3.85mm,{Triangle Cap[reversed,cap angle=120]}-{Triangle Cap[cap angle=120]},shorten <=0.25mm,shorten >=0.21mm}] (0,0) -- (0.5,0) arc (0:90:0.5) -- (1,1);
\end{tikzpicture}
\begin{tikzpicture}
\draw[draw,line width=4.15mm,{Triangle Cap[reversed,cap angle=120]}-{Triangle Cap[cap angle=120]},postaction={draw=white, shade path={shading xsep=0cm, shading=shading1b},line width=3.85mm,{Triangle Cap[reversed,cap angle=120]}-{Triangle Cap[cap angle=120]},shorten <=0.25mm,shorten >=0.21mm}] (0,0) -- (1.6,0);
\end{tikzpicture}
\begin{tikzpicture}
\draw[draw,line width=4.15mm,{Triangle Cap[reversed,cap angle=120]}-{Triangle Cap[cap angle=120]},postaction={draw=white, shade path={shading xsep=0cm, shading=shading1b},line width=3.85mm,{Triangle Cap[reversed,cap angle=120]}-{Triangle Cap[cap angle=120]},shorten <=0.25mm,shorten >=0.21mm}] (0,0) -- ++(15:1) -- +(45:1);
\end{tikzpicture}
\begin{tikzpicture}
\draw[draw,line width=4.15mm,{Triangle Cap[reversed,cap angle=120]}-{Triangle Cap[cap angle=120]},postaction={draw=white, shade path={shading xsep=0cm, shading=shading1b},line width=3.85mm,{Triangle Cap[reversed,cap angle=120]}-{Triangle Cap[cap angle=120]},shorten <=0.25mm,shorten >=0.21mm}] (0,0) arc (180:50:2);
\end{tikzpicture}

\end{document}

First approach, with "overlapping":

Based on this answer to tikz: draw multicolor curve with smooth gradient, which relies on other two (1,2) given by Mark Wibrow, you can also achieve a color gradient with the shaping path and the commands of the Shading library (Chapter 69, p.737): left color=<color> and right color=<color> (or top color=<color> and bottom color=<color>).

For a multicolor gradient, you can define:

\createshadingfromlist{<name>}{<direction>}{<colors>}

Output shading1:

enter image description here

Output shading2:

enter image description here

Code:

\documentclass{article}


\usepackage[cmyk]{xcolor}
\usepackage{tikz}
\usetikzlibrary{decorations.markings,fadings,arrows.meta}

%https://tex.stackexchange.com/questions/137357/how-to-draw-an-arrow-with-two-colors
\makeatletter
\newif\iftikz@shading@path

\tikzset{
    % There are three circumstances in which the fading sep is needed:
    % 1. Arrows which do not update the bounding box (which is most of them).
    % 2. Line caps/joins and mitres that extend outside the natural bounding 
    %    box of the path (these are not calculated by PGF).
    % 3. Other reasons that haven't been anticipated.
    fading xsep/.store in=\pgfpathfadingxsep,
    fading ysep/.store in=\pgfpathfadingysep,
    fading sep/.style={fading xsep=#1, fading ysep=#1},
    fading sep=0.0cm,
    shading path/.code={%
        % Prevent this stuff happning recursively.
        \iftikz@shading@path%
        \else%
        \tikz@shading@pathtrue%
        % \tikz@addmode installs the `modes' (e.g., fill, draw, shade) 
        % to be applied to the path. It isn't usualy for doing more
        % changes to the path's construction.
        \tikz@addmode{%
            \pgfgetpath\pgf@currentfadingpath%
            % Get the boudning box of the current path size including the fading sep
            \pgfextract@process\pgf@fadingpath@southwest{\pgfpointadd{\pgfqpoint{\pgf@pathminx}{\pgf@pathminy}}%
                {\pgfpoint{-\pgfpathfadingxsep}{-\pgfpathfadingysep}}}%%
            \pgfextract@process\pgf@fadingpath@northeast{\pgfpointadd{\pgfqpoint{\pgf@pathmaxx}{\pgf@pathmaxy}}%
                {\pgfpoint{\pgfpathfadingxsep}{\pgfpathfadingysep}}}%
            % Clear the path
            \pgfsetpath\pgfutil@empty%                          
            % Interrupt the path and picture to create a fading.
            \pgfinterruptpath%
            \pgfinterruptpicture%
            \begin{tikzfadingfrompicture}[name=.]
                \path [shade=none,fill=none, #1] \pgfextra{%
                    % Set the softpath. Any transformations in #1 will have no effect.
                    % This will *not* update the bounding box...
                    \pgfsetpath\pgf@currentfadingpath%
                    % ...so it is done manually.
                    \pgf@fadingpath@southwest
                    \expandafter\pgf@protocolsizes{\the\pgf@x}{\the\pgf@y}%
                    \pgf@fadingpath@northeast%
                    \expandafter\pgf@protocolsizes{\the\pgf@x}{\the\pgf@y}%
                };
                % Now get the bounding of the picture.
                \xdef\pgf@fadingboundingbox@southwest{\noexpand\pgfqpoint{\the\pgf@picminx}{\the\pgf@picminy}}%
                \xdef\pgf@fadingboundingbox@northeast{\noexpand\pgfqpoint{\the\pgf@picmaxx}{\the\pgf@picmaxy}}%
                %
            \end{tikzfadingfrompicture}%
            \endpgfinterruptpicture%
            \endpgfinterruptpath%
            % Install a rectangle that covers the shaded/faded path picture.                                
            \pgfpathrectanglecorners{\pgf@fadingboundingbox@southwest}{\pgf@fadingboundingbox@northeast}%
            % Make the fading happen.
            \def\tikz@path@fading{.}%
            \tikz@mode@fade@pathtrue%
            \tikz@fade@adjustfalse%10pt
            % Shift the fading to the mid point of the rectangle
            \pgfpointscale{0.5}{\pgfpointadd{\pgf@fadingboundingbox@southwest}{\pgf@fadingboundingbox@northeast}}%
            \edef\tikz@fade@transform{shift={(\the\pgf@x,\the\pgf@y)}}%
        }%
        \fi%
    }
}
\makeatother
%-------------------------------------------------------------------------------------
%https://tex.stackexchange.com/questions/197793/how-to-draw-gradient-arrows-with-tikz
\makeatletter
\def\createshadingfromlist#1#2#3{%
    \pgfutil@tempcnta=0\relax
    \pgfutil@for\pgf@tmp:={#3}\do{\advance\pgfutil@tempcnta by1}%
    \ifnum\pgfutil@tempcnta=1\relax%
    \edef\pgf@spec{color(0)=(#3);color(100)=(#3)}%
    \else%
    \pgfmathparse{50/(\pgfutil@tempcnta-1)}\let\pgf@step=\pgfmathresult%
    %
    \pgfutil@tempcntb=1\relax%
    \pgfutil@for\pgf@tmp:={#3}\do{%
        \ifnum\pgfutil@tempcntb=1\relax%
        \edef\pgf@spec{color(0)=(\pgf@tmp);color(25)=(\pgf@tmp)}%
        \else%
        \ifnum\pgfutil@tempcntb<\pgfutil@tempcnta\relax%
        \pgfmathparse{25+\pgf@step/4+(\pgfutil@tempcntb-1)*\pgf@step}%
        \edef\pgf@spec{\pgf@spec;color(\pgfmathresult)=(\pgf@tmp)}%
        \else%
        \edef\pgf@spec{\pgf@spec;color(75)=(\pgf@tmp);color(100)=(\pgf@tmp)}%
        \fi%
        \fi%
        \advance\pgfutil@tempcntb by1\relax%
    }%
    \fi%
    \csname pgfdeclare#2shading\endcsname{#1}{100}\pgf@spec%
}
\makeatother

\createshadingfromlist{shading0}{vertical}{black}
\createshadingfromlist{shading1}{vertical}{red,white}
\createshadingfromlist{shading2}{vertical}{red,yellow,green,cyan,blue}

\begin{document}
\begin{tikzpicture}
\path[shading=shading0,shading path={draw=transparent!0,{Triangle Cap[reversed,cap angle=120]}-{Triangle Cap[cap angle=120]},line width=4.15mm,}]  (0,0) -- (0.5,0) arc (0:90:0.5) -- (1,1);
\path[shading=shading1,shading path={draw=transparent!0,{Triangle Cap[reversed,cap angle=120]}-{Triangle Cap[cap angle=120]},line width=3.85mm,shorten <=0.25mm,shorten >=0.21mm}] (0,0) -- (0.5,0) arc (0:90:0.5) -- (1,1);
\end{tikzpicture}

\end{document}

TODO:

  • Must: the line color has to be black.
  • Optional: the color gradient has to follow the direction of the arrow
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