man ImageMagick :-P
I have to say i never used IM. but this could prove helpful:
Quote:
-map filename
choose a particular set of colors from this image.
[convert or mogrify].
By default, color reduction chooses an optimal set of colors that best represent the original image. Alternatively, you can choose a particular set of colors from an image file with this option.
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Source:
http://www.imagemagick.org/script/co...ne-options.php