Creating a Winter Icon to use as a base or a Black/Tan icon when there isn't one requires quite a lot of comparisons back and forth between multiple icons
For a Green to Black Icon conversion I do the following:
First, I grab the original SPCAMO Green Chieftain Icon and the SPCAMO Green/Black Camo Chieftain icons. I then look for areas on the Green Chieftain that are Black on the Camoed Chieftain, and I use the PSP eye dropper to pick up the colors from like areas and then use the PSP "color replacer" set at tolerance of 1 to replace the colors.
I then repeat this process with different areas until I'm done.
(Again, blown up 200% for clarity)