It's pretty much been covered by the other answers, but colour scheme is often dependent upon the aircraft's role and/or theatre.
For example, in the 1970-80's the USAF used a green and brown camo for attack aircraft (for protection on the ground and for low-level flight) but used a light grey for air defence fighters. They have continued to use variations of light gray for fighters, but attack aircraft now use a scheme based around greens and dark greys; simplistically this is because the assumed theatre used to be South East Asia earlier and then Europe later. Australian tactical aircraft still use a green/brown based scheme, as that is the environment they are most likely to serve in. To be honest though, most modern combat aircraft are likely to be multi-role, so variations of grey are pretty much the order of the day!
Incidentally, the RAF have adopted a slightly different approach for some of their aircraft, using a shade known as 'hemp' for some larger aircraft, on the basis that it will blend better with concrete runways and taxiways.