You want to mod FactionMapColorDefs.xml
ColorDefs.xml defines (btw, it's Definitions, not Defaults, AFAIK) the colours in the games - if you want to add new colours, you edit this.
FactionMapColorDefs.xml defines what colours can be used on the map (ie, what you want). If a colour doesn't already exists, you'll need to add it to ColorDefs.xml too. You can check out my Map Colour mods if you want to, it's fairly simple.
Sorry, it isn't colorsdefaults I should have capitalized to make it clear. It's ColorsDefAlts that I was referring to. Thats the file that has all the colors that you can actually select for the map. If you want to give a custom race say the Corn Flower Blue color, it's in THAT file as AltCornFlowerBlue. This is the file I made a mod for and it doesn't seem to work.
I'll try adding those colors to those other files and see if anything sticks though! I'm guessing I was just missing the factionmapcolordefs.xml entries.
Yep, that is what I was missing, thanks for pointing me at it!