Izzy, welcome to the forum.
I usually start with the lightest colour first, so in the case of Luftwaffe aircraft, the RLM 65/76 should applied to the underide of the wings, rear stabilisers, and the bottom and sides of the fuselage.
I then mask the lighter colour, and apply the next lightest colour, and so on until I reach the darkest colour last. The reasoning behind this approach is that it is far easier to cover a light colour with a dark, than the other way around, especially where yellow is concerned.
Hope this helps.
Karl