Stupid firefox. I hate how it deletes text fields when you go back. Guess I'll have to type this all again:
Green: As you mentioned, grass is an obvious reason. Also, green and yellow tend to look really similar, especially in darker shades (see attachment in this thread:
https://dplogin.com/forums/index.php?topic=14728.20). Some players have mentioned being red-green color blind. If we had red and green teams, they wouldn't be able to distinguish between them.
White: Already used for neutral things like observer and center flag. It would also mean we wouldn't want to put white on the other colored player skins, really limiting the designs.
Black: Lots of issues with this one. They would be very difficult to see in shadows. Current player skins already have a lot of black. You know how the blue players on pbcup_sassault are almost black because of the colored lighting? Black jerseys would be difficult to see like that all the time. It also breaks the one-letter representation of teams, "b" could be black or blue.
Plus, do we really need new colors? That would require a major revamp on the game code. Everything is hard coded for the current colors. There would need to be new textures and maps made to make use of the colors, and what would it get you?