I think I found a small glitch in the doc pgfmanual.pdf. In "86 Extended Color Support" one reads:
If you nest colormixin environments, the diff erent mix-ins are all appended. For example, inside the inner environment of the above example, \pgfuseimage{foo} would fi rst check whether there exists an image named foo.!50!white!25!black.
The example is:
\begin{colormixin}{25!white}
washed-out red text,
\color{blue} washed-out blue text,
\begin{colormixin}{25!black}
dark washed-out blue text,
\color{green} dark washed-out green text,%
\end{colormixin}
back to washed-out blue text,%
\end{colormixin}
...and I would expect that LaTeX looks for a file named "foo.!25!white!25!black".
