Probably in fact because messy paint schemes make better camouflage. At the start of the war, the 70/71/65 scheme came with "hard" straight edges. The pre-war three colour scheme was equally sharp-edged. Nothing in nature is hard or straight or sharp like that, and while the idea of breaking up the shape of the aircraft was good, the lack of curves and tonal difference made the camouflage useless. Then came the use of 02 instead of 70 - better tonal differences, so more disruptive effect - and mottling on the fuselage. Then 74/75/76 for higher altitudes, just as the RAF switched to Ocean Grey instead of Dark Earth for the Temperate Day Scheme. Etc, etc. Each step takes us closer to an effective camouflage scheme. Don't forget that the scheme has to be effective at 20,000 ft and on the ground... Of course, as the War became more difficult for the Luftwaffe, changing tactical situations and failing logistics obviously played a part, but the trend had already been established earlier on.
It is interesting to note that bombers did not follow quite the same evolution, and that the factory schemes on heavy aircraft tended to stay relatively unchanged; overpainting was done on-squadron, rather than before delivery.
The RAF never suffered from the confused logistics the German had to put up with at the end, but anyone who thinks that RAF aircraft were all uniformly painted has missed it. There were strict orders concerning the layout of colours, and largely, the colours used were appropriate, which was not the case of the Luftwaffe at the outset. However, if you follow the development of the colour of the undersides of RAF day fighters from late 1939 until mid-1941, you will see a plethora of odd colours being used, particularly during the Battle of Britain. When Sky was introduced as an official MAP colour, it was not available. Manufacturers and MUs used a multitude of other similar colours such as Sky Grey and Eau-de-Nil to make up the shortfall; as a result, you could find some half-a-dozen different shades in use at any one time. By late September 1940, supplies of Sky were available... Much the same problem arose when grey was introduced to replace Dark Earth in the Temperate Day Scheme; the new grey paint - to become Ocean Grey - was mixed on-squadron out of seven parts of Medium Sea Grey to one part of Night. The resulting shade was extremely variable, although in general darker than the standardised MAP Ocean Grey which appeared on production lines by mid-1942. Because the layout of the paint scheme was unchanged, it is less obvious to the untrained eye.
The Americans had a rather different situation; by 1943, which is when their presence actually began to be felt, the RAF had already established local air superiority over Great Britain and Northern Europe. That alleviated the need for such effective camouflage on USAAF aircraft, as the likelihood of being attacked on the ground was far lower. Interestingly, the USAAF never really bothered about effective high-altitude camouflage... By the time the Americans had got around to making their presence felt, the Luftwaffe was no longer quite the force it had been... Unless you were flying bombers into the very heart of the Reich. But then, the sheer numbers of aircraft involved made any attempt at discretion relatively pointless!
