i can never decide what colors a map should be, i always end up with a generic-gray bacground and too much:
green
white
gray
black
really really bright colors
nuclear power green

any help? how do you decide what colors?

It depends on what kind of mood you want to convey, as well as what the environment itself is.

Naturally, you want the colors to appear natural. Never use any truly "pure" color (as in red, blue, etc.). Even if it's a man-made , make it look like it's dirty or something. Also keep in mind any light sources--whether it be the sun, fiery pools of magma, or fluorescent lighting. Large groups of polys should be darker towards the center too, of course; it's much more aesthetically pleasing.

Also, colors have a huge impact on mood. Deep greens and blues are "cool" colors, and are relaxing, whereas red and orange are "warm" and induce excitement. White tends to convey a feeling of emptiness and of purity, while black usually represents fear, death, and corruption. Red is another color representative of death, and also induces hunger and anger... You can probably find a whole list of these types of relationships if you Google it.

Lastly, make background objects lighter, and get progressively darker as they get "closer" into the foreground.

Hope that helps to an extent :)

If you want it to be post-war environment go with an Indian red high, something like MS_Invaded and transition to gray, I think.

If it's jungley make the background aquamarine, or greenish, like Cambodia and Shau, Tropiccave. If it's desert-like or the map resembles something like WWI trench warfare put dark-reddish colors, like inf_Outpost; almost never use anything bright (the BG colors combine tho' so I guess some). Bright BG colors should be reserved for urban combat, and even then they shouldn't be too radical.

You want to usually convey a darkish atmosphere; the game's all about massacre, man xP

grass:
64,128,64
96,128,96
112,128,80

clay (for underground):
128,64,64
128,96,96
160,96,80

red base:
160,128,128
80,64,64

blue base:
128,128,160
64,64,80

bluestone:
104,104,128
80,80,96

mess with RGB a bit.

by the way, from nothing to full is:
0,64,128,192,255
0,32,64,96,128,160,192,224,255
0,16,32,48,64,80,96,112,128,144,160,176,192,204,224,240,255