Just starting with the painting.
I primer coat using white, then pre-shade using a "complimentary colour" to the camouflage. In the case of the 3 RAF aircraft, Spitfire, Hurricane and BP Defiant, I have used RLM 70 Blackgreen. For the Bf109 I have used RAF roundel blue, and dark earth, for the yellow nose areas.
I sort of cocked up the method when applying the under surface colour (Sky) on the RAF machines in that I should have gone over the RLM 70 pre-shading first, then paint along side it, before several coats that cover everything. The slight hiccup doesn't matter though as I painted over the RLM 70 second. The trick, for me anyway, is that when you think it is close to being the right amount of paint stop. Come back the following day and you will, more than likely, see that it wasn't quite enough so add more, carefully and stop and come back again, if necessary. The Bf109 was done in the same way but I remembered what I was supposed to be doing but, like I said, it didn't really make any difference
Just to explain a little more, after painting over the pre-shading then along side it spray a little wider covering both areas. The idea is to leave the white primer coat unpainted in the centre of panels so that when you apply the "blending coat" (wider pattern and overall coats), the darker pre-shaded areas get a bit more coverage and the lighter areas show through the paint to make those areas just a tad lighter. ISH!