Change colour depending on counter

Question

I have some text that I want to change colour depending on the current state of a counter. But I can't get this file to compile. (This is obviously simplified, since the texcolour command is actually part of a command defined in the preamble...)

\documentclass{article}
\usepackage{xcolor}
\newcounter{foo}
\setcounter{foo}{30}
\begin{document}
\textcolor[RGB]{0,\value{foo},0}{Text in FOO colour}
\end{document}

I get the following error repeated:

! Missing number, treated as zero.
<to be read again> 
                   \
l.8 ...r[RGB]{0,\value{foo},0}{Text in FOO colour}

! Illegal unit of measure (pt inserted).
<to be read again> 
                   \
l.8 ...r[RGB]{0,\value{foo},0}{Text in FOO colour}

I'm sure there's some subtlety I've not grasped yet... But \vskip \value{foo} pt compiles fine and gives me space depending on the value of the counter, so I can't be far off...

Answer 1

Insert \the before \value; then it works.

As to why you have to include \the: The culprit is \@onelevel@sanitize that is called inside \XC@edef. What?? Let me try and explain. At some point, your \textcolor command makes the call

\XC@edef\@@clr{0,\value{foo},0}

This in turn calls

\edef\@@tmp{0,\value{foo},0}

(with some trickery regarding active characters), thus making \@@tmp contain 0,\c@foo,0 and not 0,30,0: The \edef doesn't expand the counter \c@foo to it contents. Immediately after this, \XC@edef calls

\@onelevel@sanitize\@@tmp

which in turn expands to

\edef\@@tmp{\expandafter \strip@prefix \meaning \@@tmp}

And this does something horrible: It turns the 5 tokens 0,\c@foo,0 into the 11 tokens 0,\c@foo ,0, i.e., the control sequence \c@foo is turned into a sequence of single characters (all with catcode 12, except the space). Now you've got your explanation for the strange error message

! Missing number, treated as zero.
<to be read again> 
                   \

The \ is the first character of \c@foo! And it's not a number, indeed.

If you could follow up to this point, then it's easy to understand why \the helps out here:

\edef\@@tmp{0,\the\value{foo},0}

makes \@@tmp contain 0,30,0, and no evil can happen anymore.