[pgf-tikz] RGB colour model for plain TeX users

[Jim Diamond]

On Sat, Nov  9, 2019 at 11:30 (+1300), Henri Menke wrote:

> Dear Jim,

> Thanks for the suggestion.  I've opened a new issue on GitHub for it
> and added it to the roadmap for the next release.

> https://github.com/pgf-tikz/pgf/issues/772

Hi again Henri,

I will also put this on github, but a couple of minutes of playing
around gave me

\def\pgfutil at emu@cmyk#1#2,#3,#4,#5\@nil
{
    \pgfmathsetmacro{\@red@}{(1 - #2) * (1 - #5)}%
    \pgfmathsetmacro{\@green@}{(1 - #3) * (1 - #5)}%
    \pgfmathsetmacro{\@blue@}{(1 - #4) * (1 - #5)}%
    \expandafter\edef\csname\string\color@#1\endcsname
    {\noexpand\xcolor@ {}{}{rgb}{\@red@,\@green@,\@blue@}}
}

I realize that converting cmyk to rgb is a lot more contentious than
converting RGB to rgb, but in case anyone aside from me thinks this is
more useful than harmful, ...

Cheers.
                                Jim