"I cannot consider it a science when in most cases no science methods are applied."
Certainly true when modellers speculates and questions specific schemes of their chosen subjects. But those who study the subject seriously (like Ullmann, Merrick, Wadman to name a few) certainly applies scientific methodology.
"Most colourful paint schemes are actually an effect of misinterpretation of photos, lack of knowledge and wishful thinking."
I wouldn't say most, but admittedly there have been many imaginative schemes proposed over the years. However, because of the scientific method now being applied the majority of these have now been dismissed. Sadly, they often live on in the minds of many modellers and enthusiasts through older books and magazines. Examples include "Graff's all-yellow Fw 190", Fw 190s with red cowls, etc.
German colours are one thing, the ones used for upper surface ccamouflage were in my view rather few, although tonal variation no doubt varied a lot. They usually consisted of the regular greys (74 & 75) and a few others (79, 80, various greens and a few browns, both mixed and official).
It is when we consider the application of these colours that the true imagination and creativity of the Luftwaffe painters becomes obvious. Even you Franek, must admit that the variation in application style is way beyond that prescribed or intended by the RLM!!!?
And Luftwaffe certainly had nose art and fuselage art almost as gaudy as that of the Americans. Just consider the Ju 52 with the fuselage snake marking (Luftwaffe im Focus # 4, p. 24-25) or the Stuka with its snake (Luftwaffe im Focus # 7, p. 24-27)! They are not imagined.
One thing that fascinates me, is how you in many cases can actually recognize the unit to which a Luftwaffe aircraft (mainly fighters) belongs from its camouflage style alone, even when no unit insignia are visible! Try to do that with an RAF Spitfire (and please, this is not a bait to get a "Luftwaffe is so much better" discussion going, just a factual observation)!!
As an example of such unit specific camouflage, consider this heavily repainted Bf 109F-4/trop from the 10xxx block delivered to the Luftwaffe (specifically 6./JG 5) some time in 1942:
Kjetil