Aircraft were painted a color that blended with whatever background in WWII. To eliminate confusion with the red "meatball" of the Japanese, two white lines were added and the red center eliminated from the pre-war insignia star. Other allied forces also used this in the Pacific, the Brits added bars and eliminated the red in their round insignia as well, but they only did it in the Pacific.
Anyway, as we gained air supremecy, and production sped up, aircraft left the factory without paint except for insignia and anti-glare panels. That's why you see late war B-17's, P-51s and the like looking silver. They simply didn't have paint.
The Air Force kept the "no paint" scheme through Korea and into the cold war. In Korea, stripes were added to the fuselage of F-86's to make it easier to diferentiate between an F-86 and Mig-15, as they looked very similar. You may remember sivler B-36's, 47's or 52's from these days. The Navy went with the white and color. All services adde a red stripe to the white bars.
During Vietnam, the Air Force went with a camo color, but retained some coloring for emergency equipment (like canopy release outside, etc). The Navy stayed with white. Army aircraft were painted green, same as a truck and got rid of national insignia, leaving just "United States Army" in letters.
After Vietnam, as new technology developed, it was found that newer threat IRdetectors could pick up infra red signatures on color painted aircraft quicker than they could on non colored aircraft, especially the yellows and reds used to mark engine intake and other safety markings. An F-14 painted white and all sorts of colors is detected sooner and at longer range, than an F-14 that's painted the current haze gray. At this time the Army switched to CARC paint on tactical vehicles, which resits NBC contamination. CARC is used on trucks, tanks, etc and comes in all colors, but the Army only allows certain shades of "blackish" to be used on helicopters due to IR suppression. In fact which "blackish" paint to use depends on which airframe it is. The paint used on an AH-64 is actually different than used on a UH-1H for example, due to the way the IR suppression works in conjunction with the other systems and the signature of the aircraft.
Needless to say, alot of work goes into why aircraft are painted the way they are. It's not a matter of "cheaper" or "looks better" or any of that. There's a BUNCH of highly technical reasons for that paint being there.
Ross