\externalfigure has a "color" parameter. It used to be possible to colorize greyscale pictures with that, but it doesn’t work any more:

%%%

\setupcolors[state=start]
\setupexternalfigures[
    location={local,global,default},
]

\starttext

\externalfigure[img/minidodo_g][width=.3\textwidth,color=red] % greyscale
\externalfigure[img/minidodo_b][width=.3\textwidth,color=green] % bitmap
\externalfigure[cow][width=.3\textwidth,color=blue] % vector

\stoptext
%%%

Greetlings, Hraban
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