The standard color scheme for US Fighters was olive drab over neutral gray. This was considered my most to be the most effective overall camouflage color since no one could guarantee what type of terrain these aircraft would operate over. OD in the South Pacific was effective at low levels, over Italy, not so much.
The US switch from OD to natural metal came about as a result of three things happening - pilots learning that aircraft stripped of paint and polished were faster, the overall high altitude in which escorts were operating, and the overall air superiority the Allies achieved shortly after D-Day. That's not to say that the Luftwaffe rolled over on its back and wet the floor, quite the contrary. But with the exception of Bodenplatte at the end of the year, the Luftwaffe never really mounted large-scale offensive operations against the Allies.
Fighter camo in the desert regions was largely based on RAF doctrine of Dark Earth and Midstone over Sky Type S or Duck Egg Blue. Remember too, that many of the P-40s that were rushed to North Africa for the Torch Landings were originally intended to go to the RAF. I have photos somewhere showing P-40s on the assembly line with RAF markings being painted out with a star and roundel.
D-Day stripes were an anomaly - they were intended to be temporary. Most squadrons, especially those involved in escort and fighter sweep duties, lost their stripes with a few months of the order. Some retained them longer. A few removed the stripes on the upper sides and kept them on the bottom, since the Allies now had a large ground contingent on the Continent. Prior to that, getting shot at by your own flak wasn't a big deal - there was none prior to the invasion.
Overall, the Luftwaffe exercised very little control over the use of camouflage on its aircraft, especially day fighters. That's why I have 4 or 5 books on US camo doctrine and more than 60 on Luftwaffe colors in my library! Luftwaffe camouflage practices are a science within itself, especially when you start looking at the 1944-45 period of time when RLM 80, 81, and 82 were introduced, and the subsequent usage of those colors in conjunction with the earlier RLM 74, 75 colors.
Luftwaffe desert camo is surprisingly a bit more simple. When JG27 arrived in North Africa, they actually painted out their 109s in Italian stocks, as RLM 79 hadn't been delivered to the front. In most cases, you'll see JG27 aircraft carrying a motte pattern of RLM 71 or RLM 02 over Italian Sand, a few months later, photos will show these same aircraft carrying RLM 71 or 02 over RLM 79. Unlike their Regia Aeronautica allies, German camo patterns ranged from overall sand, to motte (soft spots), waves, and splinter patterns.
J