[Tex/LaTex] Understanding xcolor color mixing model

Question

I fail to understand the color mixing of xcolor.

I want to have a lighter and a darker version of a color defined as

\definecolor{BlueLUH}{cmyk}{1.0,0.7,0,0}

My approach of lighter and darker is this one

\colorlet{LightBlue}{BlueLUH!20!white}
\colorlet{DarkBlue}{BlueLUH!80!black!20}

which according to the docs is replaced internally to

\colorlet{LightBlue}{BlueLUH!20!white!white}
\colorlet{DarkBlue}{BlueLUH!80!black!20!white}

Now I wonder what is actually calulated, bcause I get the same for Light and Dark, as can be seen in the following screenshot:

Answer 1

That are different colors, but cannot be seen! 20% of black is less than light gray! Compare it with a 40% of black! The values for the cmyk color can easily be seen in the pdf output when using \pdfcompresslevel=0.

The colors are calculated as:

\pdfcompresslevel=0
\documentclass{article}
\usepackage[cmyk]{xcolor}

\definecolor{BlueLUH}{cmyk}{1.0,0.7,0,0}
\colorlet{LightBlue}{BlueLUH!20!white}
\colorlet{DarkBlue}{BlueLUH!80!black!20}
%\colorlet{LightBlue}{BlueLUH!20!white!white}
%\colorlet{DarkBlue}{BlueLUH!80!black!20!white}

\begin{document} 

\color{BlueLUH}\rule{1cm}{1cm}
\color{LightBlue}\rule{1cm}{1cm}
\color{DarkBlue}\rule{1cm}{1cm}

\color[cmyk]{1, 0.7, 0, 0}\rule{1cm}{1cm} 
\color[cmyk]{0.2, 0.14, 0, 0}\rule{1cm}{1cm}
\color[cmyk]{0.16, 0.112, 0, 0.04}\rule{1cm}{1cm}

\colorlet{DarkBlue}{BlueLUH!80!black!40}
\color{BlueLUH}\rule{1cm}{1cm}
\color{LightBlue}\rule{1cm}{1cm}
\color{DarkBlue}\rule{1cm}{1cm}

\end{document}