Seems to me we only need to make a few changes in grdimage to allow an image with no colormap use a CPT and pick colours from it depending on the byte values. Looking at this now. The only case we have is byte image with the bytes being treated as gray (sadly 0-8) but if we pick from CPT instead I think it will work OK. Just need to ensure the output image is 3 layers while input is 1.