Yes, you understood correctly — the terrain designation on the minimap. Even if it's a color fill, there must be files in the game with that fill. That's my assumption. But I couldn't find them in the game. I don't know where to look.
whoa.... no, I don't know that for sure
the only thing I can try to suggest is to take a look (if you haven't already) in the "data.pak\MapObjects\_(AdvMapTile)" folder — essentially all the texture tiles that can appear on the minimap are there, and in every .xdb file there's a line "<MinimapColor>". only there are very strange numbers after those words
so I'm not at all sure that's the right thing. but if there are no other options yet, and the people in the know are asleep, you can poke around there if you want 
UPD: ha, lol, it is that thing
I got curious and decided to poke around there myself for fun. now all the grass on my minimap is blue