When the US first joined the war, their planes were painted green. By the end of the war they didn't seem to bother with anything aside from serial numbers/ insignia, leaving the planes bare aluminum. What was the reason behind that?
The October 1943 directive to stop painting aircraft had several benefits. First of all, the paint itself as a substance is quite heavy (more so than water, at about 8 pounds per gallon) When it is applied to the plane it adds potentially several hundred pounds of weight in the case of a B-24 or other large bomber. The weight and drag (friction) of a painted skin compared to a bare metal skin actually causes a noticeable difference in speed. Skipping the paint shop also speeds up the production and hence delivery of aircraft. The primary purpose of camouflage is to keep planes hidden on the ground; by early spring 1944 when the first unpainted aircraft began arriving in England, the Americans had complete air supremacy over the British Isles and the threat of intruding German aircraft was nearly nil.
This created an interesting mix of bare metal and camouflaged aircraft for a while, but as the spring of 1944 wore on and attrition took its toll, the number of camouflaged aircraft dwindled. By the summer, most B-17 groups consisted of entirely bare-metal aircraft, and with the influx of many new bomb groups, many more variations of the mandated brightly colored identification markings were adopted
One of my favorites, the 4th Combat Wing (in this case the 447th BG), with their bright yellow tails and wing tips, fuselage stripes, and wing "V". The three other groups in the 4th Combat Wing also had yellow tails and wing tips, but different variations and colors of fuselage stripes and wing "V"'s
Interestingly, many units actually applied full or partial camouflage to their bare metal aircraft once they received them. This paint was often locally "obtained" in the middle of the night from nearby RAF bases! In some cases, the exact color is difficult to determine. The 78th Fighter Group sometimes used a two-tone pattern of British Dark Green and Ocean Grey over Sky (a light greenish cream color) In anticipation of being deployed to the continent (meaning France) after D-Day, the 20th Fighter Group painted their new P-51 Mustangs' wings and upper fuselages in a dark green color. This never materialized and the paint was mostly stripped off by the end of the year.
In an extreme example, when the hot rod P-47M was received by the 56th Fighter Group, the whole concept of "camouflage" went out the window. The 61st Fighter Squadron painted their planes a deep purplish-black, the 62nd Fighter Squadron went with a dark green and gray, and to round it off, the 63rd FS and their two-tone blue planes with flashy bare metal highlights.
Lt. Leo F. Butiste's P-47M 44-21199 LM:X, 62nd FS, 56th FG
A period photo, and then modern recreation of Capt. Jack M. "Happy Jack" Ilfrey's P-51D 44-13761 MC:I, 79th FS, 20th FG. In period photos, the green seems much darker, almost like RAF Dark Green, than the more brownish color of the restoration, but then again, the exact identity of the paint is lost to history
Sources:
Aircraft of the 8th AAF 1942-1945, by Kev Darling
20th Fighter Group
http://www.littlefriends.co.uk/gallery.php?Group=20
56th Fighter Group
http://www.littlefriends.co.uk/gallery.php?Group=56
78th Fighter Group