255, 256, whatever
what do u mean by color index numbers?
Like, the first color in the pallet is number 1, the second is number 2, the third is number 3...
I think the deal with decals is that the last color value is the decal color (red, green, yellow, ...) and the others are for defining the transparency of the texture (1st being the most transparent amd 254th being the less transparent.)
tbh, I don't have a clue is the thing really like this but it seems to be the most logical way to me.