Hi, Amiers...
As I understand it, the yellow cowlings, wingtips, tailplanes etc were a means of quickly identifying friendly aircraft, and were used for the express purpose of 'standing out like a sore thumb', if you like. If a Bf109 was in a dogfight during the Battle of Britain, for example, it would have been an easy way to distinguish your fellow countrymen flying past at relatively close quarters, perhaps even unexpectedly. German bomber crews would have been able to tell the difference sooner whether or not the small black dots in the distance which were rapidly catching them up, were the escorting Bf109s or the attacking RAF fighters, and could act accordingly.
During the invasions of Yugoslavia and Greece, it should be remembered that both those countries had received Bf109s and Dornier Do17s (Yugoslavia) and Henschel Hs126s (Greece) from Germany prior to those campaigns, so again, from a distance, the German machines would have been recognised by their countrymen more easily, and the same would go for the early battles in the soviet Union.
Naturally, as the war deteriorated for Germany, the rapid I/D colours were largely dispensed with. I think there was always a need for some sort of easy recognition, hence the colourful
Reichverteidigung bands on the home defence units, and on a couple of machines I can think of, an Fw190 and an Me410, completely white vertical tail surfaces. Even in the final months, I believe an order went out to air force units to clearly mark their machines with yellow once again. I know of one Fw189, 5H+RK, W.nr.125715, of Nahaufklärungsgruppe 16, (page 108 of
Aufklärer by David Wadman, John Bradley and Barry Ketley - Hikoki Publications
) with rudders, cowlings and quite possibly upper and lower wintips in yellow.
Germany wasn't alone in this pracitce though, but application differed, if you like. Japanese aircraft for example, almost always had a rapid I/D yellow strip along the wing leading edges, and presumably the aircraft of Vichy France were intended to be seen with their not-so-shy-and-retiring bright red and yellow engine cowlings.
Regards...