Warlord games painting guide for WW2 figures.
At the simplest level: US: green or khaki, Germans: grey (feldgrau), Soviets: brown tending toward rust. British: tan. That said, the different branches of each service and different periods of the war, in addition to different levels of field use makes the colors quite diverse. Wargamers often have strongly-worded discussions on uniform colors. Warlord Games has lots of resources on this.
You can also follow the unofficial but very good Memoir 44 painting guide, which may be more in line with what you are looking for, if you play Memoir 44.