Maybe this can help :
The following command finds a colormap of the most different colors than the one present in the input image.
Use it like this: $ gmic sp lena +non_colormap 16.
non_colormap : check "${1=32}>0"
e[^-1] "Compute a $1-entry colormap of the most different colors from image$?."
v - repeat $! l[$<]
64,64,64,1 eval.. "col = [R,G,B]*(w#-1-1)/255; i(#-1,col) = 1" rm..
$1,1,1,3
repeat w # Find colormap by maximal color distance from existing points
+distance.. 1 col={[xM,yM,zM]} rm.
point.. $col,1,1
point. $>,0,0,1,{[$col]/w#-2*255}
done rm..
l. +luminance. rv a y sort. +,x rows. 1 round endl # Sort colormap by increasing luminance
endl done v +