The problem is that the \begin{...} and \end{...} pair commands automatically create a "group" so that, in effect, the & and \\ are "out of scope" for the tabular, while inside the "production" environment they just show up at a place where the compiler is not expecting them.
A second problem with your definitions is that, even if they would work, they would be adding an extra \\ at the end of the tabular, adding an unwanted space at the end. Perhaps some more appropriate definitions would be
\documentclass[a4paper,10pt]{article}
\newcommand{\production}[1]{#1 ::= &}
\newenvironment{grammar}{\tabular{p{3cm}l}}{\endtabular}
\begin{document}
\begin{grammar}
\production{XmlStartTag} ... \\
\production{XmlOtherTag} ... \\
\production{XmlEndTag} ...
\end{grammar}
\end{document}
Note no \\ at the end of the last production. Also in the definition of the grammar you don't need to repeat the work of \begin/\end, and you can instead directly use \tabular and \endtabular.
I'd use the xparse package to do this, as everything is then 'pre-packaged':
\documentclass{article}
\usepackage{color,xparse}
\NewDocumentCommand\MyTextColor{m+g}{%
\IfNoValueTF{#2}
{\color{#1}}
{\textcolor{#1}{#2}}%
}
\begin{document}
\MyTextColor{green}{stuff}
\MyTextColor{red} Some text
\end{document}
The same can of course be done using \@ifnextchar
\documentclass{article}
\usepackage{color}
\makeatletter
\newcommand*\MyTextColor[1]{%
\@ifnextchar\bgroup
{\textcolor{#1}}
{\color{#1}}%
}
\makeatother
\begin{document}
\MyTextColor{green}{stuff}
\MyTextColor{red} Some text
\end{document}
The key point is that you need to look for { using \bgroup rather than trying to use it directly (as it is the begin-group character).
To explain how the \@ifnextchar part deals with the need for either 1 or 2 arguments, what happens in my second approach is that
\newcommand*\MyTextColor[1]{%
defines a macro which absorbs one argument. So:
\MyTextColor{red}{stuff}
absorbs red as #1 and leaves {stuff} in the input stream, while
\MyTextColor{red} other stuff
also absorbs red as #1 and leaves other stuff in the input stream. We now apply \@ifnextchar, which 'peeks' at the next token without absorbing it. The test reads
\@ifnextchar\bgroup
{\textcolor{#1}}
{\color{#1}}%
and so depending on the result, either \textcolor{red} or \color{red} is inserted into the input stream. Thus we end up with either
\textcolor{red}{stuff}
or
\color{red} other stuff
which means that \textcolor will get the required two arguments, while \color only gets one. The key is that {red} is reinserted into the input after either \textcolor or \color.
Although it is not directly related to the question, a few notes on the xparse approach may be useful. When using the xparse package, arguments are described by letter, with m representing a mandatory argument and g representing an optional argument in braces (i.e. an optional TeX group). A + before the letter allows that argument to accept paragraph tokens, so in my definition the first argument cannot (it's supposed to be the name of a colour), while the second one can (it is arbitrary text).
The g argument returns a special token (\NoValue) if the optional argument was not present at all. If a default value was wanted, I'd have used G{<default>}. The same approach applies to standard LaTeX optional arguments, which are represented as o or O, depending on whether a default value is required. I have tested for \NoValue with the test \IfNoValueTF; \IfNoValueT and \IfNoValueT are also available.
Best Answer
Based on details from How to iterate over a comma separated list?, you can define your versions of
\pageand\pagesin the following way:\pagepasses its argument on to making a\definition called\@page.\pagesiterates over the CSV list, passing every item to\@pagefor execution.