I think it was due to "elite" status and technological progress. During WW1 mostly prestigious families were assigned to be pilots. They did color their planes as they wished once they reached certain status (Red Baron for instance). They didn't care that much about camouflage either, they were slow and noisy. Easily spotted from far and with plenty of time for reaction. But this was not much of an issue because air defense was very lacking. Most of these planes were brought down in air-to-air combat.
During WW2 it became common practice. The number of airplanes was in the thousands. To be a pilot was prestigious but they came from all types of families. Since they were running on large squadrons they needed to follow the same rules for everyone. You could decorate your plane with "logos" and stuff, but not so distinct as to not even know which was your squadron. Airplanes were still noisy but they were way faster. Camouflage became more important also since sound travels slower than light. You could see them from far but the moment you hear them to actually bombing you, was drastically reduced. Allowing one airplane to be red or pink would place you and all your squadron in danger if seen from the distance. Ground-to-air defenses were much better now, they didn't need to launch airplanes and wait as before. With a little warning, they could bring you down from the land.
That's my guess