Seriously: how often do we say "look out for the blue army!" We usually refererence sides by name. This would have the nice side effect of helping out any color-blind players.
I say "watch out for <color>" in strategy games ALL THE TIME! I'd almost say its a flaw with the game "demigod" because the colors are so subtle that I can't do it. I have to try to spell their crazy names. I've encountered 2 color blind gamers, only 1 of which couldn't figure out what player I meant at any time. (this is in part because I'm always red, and I wouldn't refer to myself by color, and the rest could be distinguished)
This is because there are usually so many colors on the map that saying "tan" doesn't help. In Master of Magic, Sins of a Solar Empire, age of empires, starcraft, and risk I almost exclusively refer to players by their color rather than their actual faction names, unless I'm in a group of particularly good friends. Dawn of War I often refer to by their color scheme. In age of empires 3 and Civ I refer to them by their AI player in cases where it applies. Civ 4 I sometimes say "watch out for" and use the type of civilization or the leader, but I still use color from time to time.
If I saw a bunch of "blue" an my mini-map I'd blame "blue" if I wasn't expecting that kind of trickery. Especially with custom armies, since they wouldn't look that different otherwise.