Confused about mixing colors in tikz / xcolor

Question

I noticed first that yellow!50!green did not produce the same color as green!50!yellow. Then I tried more systematically:

\begin{tikzpicture}
\foreach\y in {0,10,...,100} {
\fill[green!\y!yellow] (.1*\y,0) rectangle ++(1,1);
\fill[yellow!\y!green] (.1*\y,2) rectangle ++(1,1);
}
\end{tikzpicture}

This is what I get:

What exactly is going on there?

Answer 1

1. xcolor uses color model natural by default.
   (See xcolor package doc, Table 1: "Package options".)
2. In natural color model, green is in rgb model and yellor is in cmyk model.
   (Can only be checked clearly from source code (1 and 2), but the Figure 4: "Target color model" in package doc tells the difference a bit.)
3. In a color mix expression like <name1>!<pct>!<name2> (<pct> is short for percent), if <name1> and <name2> are in different color models, in normal case when 0 < <pct> < 100 the resulting color has the same model with <name1>.
   (See xcolor package doc, sec. 2.3.2 "Meaning of standard color expressions", the note paragraph after numbered list.)
4. Specifically, if <pct> is 0, <name2> is returned, without model transformation. In this case, the resulting color has the same model with <name2>, not <name1>.
   (ditto)

To set or change current color model, one of the following ways can be used:

• Use \PassOptionsToPackage{<model>}{xcolor} before xcolor is loaded.
• Use model name as package option, for example \usepackage[rgb]{xcolor}.
• Use \selectcolormodel{<model>} after xcolor is loaded.

\documentclass{article}
\usepackage{tikz}
\newcommand{\printColorSpec}[1]{%
  \textcolor{#1}{\rule{1em}{1em}} 
  \extractcolorspec{#1}{\temp}%
  \makebox[4cm][l]{\detokenize{"#1"}} ->
  \detokenize\expandafter{\temp}\par
}
\newcommand\testColorMixing[1]{%
  \noindent Color Model: \texttt{#1}\
  \selectcolormodel{#1}%
  \begin{tikzpicture}
    \foreach \y in {0,10,...,100} {
      \fill[green!\y!yellow] (.1\y,0) rectangle ++(1,1);
      \fill[yellow!\y!green] (.1\y,1.1) rectangle ++(1,1);
    }
  \end{tikzpicture}
  \par
}
\parindent=0pt
\begin{document}
Color Model: \texttt{natural}
\selectcolormodel{natural}
\ttfamily
\printColorSpec{green}
\printColorSpec{yellow}
\medskip\hrule\par\medskip
\printColorSpec{green!0!yellow}
\printColorSpec{green!10!yellow}
\printColorSpec{green!20!yellow}
\hspace*{.3em}\vdots\par
\printColorSpec{green!90!yellow}
\printColorSpec{green!100!yellow}
\medskip\hrule\par\medskip
\printColorSpec{yellow!0!green}
\printColorSpec{yellow!10!green}
\printColorSpec{yellow!20!green}
\hspace*{.3em}\vdots\par
\printColorSpec{yellow!90!green}
\printColorSpec{yellow!100!green}
\medskip\hrule\par\medskip
\testColorMixing{natural} % default
\testColorMixing{rgb}
\testColorMixing{cmyk}
\end{document}