The Airacobra I was the British equivilant of the P-39C, with a slightly more powerfull engine, a 20 mm in place of the origional 37 mm and 6 .303 mgs (2 in nose and 2 in ea wing) - The British flew on op with 4 of them and then decided there was no place for the Airacobra in the RAF. Of the balance of the contract, 212 were diverted to the Russians, with the USAAF taking 179. 100+ were shipped to Australia, the majority ending up in New Guinea and Guadacanal. Apparently most, if not all were painted in RAF camoflage (Dark Green and Dark Brown uppers and Sky lower surfaces) and serials when they got to Australia, with USAAF corcardes replacing the RAF roundels and U.S. ARMY under the wings (and 30 cal Brownings replacing the .303).
Apparently, the aircraft were slowly repainted in OD and Neutral Gray. However in any pictures of P-400s on Guadacanal I have seen, they were definatly in a complete RAF scheme. In the pictures of P-400s operating in New Guinea I have seen, they've been pretty evenly split between the RAF and USAAF schemes.
I've seen a couple of after market (and one old P-39 kit instructions) that state that the aircraft undersides were neutral gray. Everything I've seen so far would indicate that if the aircraft had RAF camo on its uppers, its lowers were Sky and if the uppers were OD, then its lowers were Neutral Gray. Apparently, when the aircraft were painted, it was at a Depot level facility and both colors were painted in the same session. (Now watch someone come up with a picture of a P-400 with RAF uppers and Neutral Gray lowers)
On the P-63, my research indicates upwards of 2400 were constructed from early 1944 to the end of the war. The vast majority going to the Soviets (up to 85 per cent by some reports) with the remaining going to the US and French Air Forces. Someplace around here I have a picture of the Anchorage, Alaska Air Force Base taken during the winter of 1944-45 and you can count 200 + P-63s awaiting delivery. I've also, over the years seen photographs of Soviet Aces operating P-63s at the end of the European war.