Doesn't work with \usepackage[cmyk]{xcolor}

[dcpurton]

See https://tex.stackexchange.com/q/435909/87678 for the question that precipitates this bug report.

Spot colours do not work if xcolor is loaded with the cmyk option. When you try and use a spot colour under these circumstances an error is produce:

! Argument of \c@lor@@cmyk has an extra }.
<inserted text> 
                \par 
l.9 \textcolor{registration}{Registration colour}

MWE

In this MWE, my test colour is RGB red and CMYK cyan, so you easily see which colour is in use.

\documentclass{article}
\usepackage[cmyk]{xcolor}
\usepackage{colorspace}
\definecolor{testcolour}{RGB/cmyk}{255,0,0/1,0,0,0}
\definespotcolor{registration}{All}{1,1,1,1}
\begin{document}
\textcolor{testcolour}{Test Colour}

\textcolor{registration}{Registration colour}
\end{document}

[jbezos]

Yes, the package option cmyk just calls \selectcolormodel, which, as explained in the manual, doesn't work yet. This is a limitation, which means cmyk must be selected explicitly when defining colors.

[jbezos]

Sadly, this is far from trivial. If, say, the color model cmyk is selected, xcolor assumes the value to be written to the PDF is always cmyk (it forces it, and even gray is converted). Changing this behavior requires patching a good deal of internal macros. For the moment, it's a limitation we have to live with.

[dcpurton]

I think it's OK. Once I figured out that as long as the current colour model is natural when a spot colour is actually used it's fine. On reflection, I think that \usepackage[cmyk]{xcolor} should have the effect of outputting spot colours in CMYK anyway (perhaps this is easier to implement?), which is different to what I was initially thinking. (I initially expected that it would output everything in CMYK+spot.)

[jbezos]

Outputting spot colours in CMYK is also difficult (I considered this possibility, too), at least if done at the time the PDF is output.