I used RGB 175,175,185 on my B-29 skins. The tutorial is just intended as a guide to doing bare metal. Don't treat it as gospel, you should always use your judgement to develop your own style.
The idea of the green and blue layers is to represent reflections from the sky and ground. So the top surfaces of the wings, tail, fuselage and nacelles etc. get a light airbrush of a blue sky colour and the lower surfaces a terrain-like green colour. (IIRC I got the colours I used from an AH screenshot). You just have to imagine which bits of the aircraft would reflect sky and which bits the ground. The white reflections are more concentrated. I usually apply them on the top of the wings and tail near the main spar, along the top third of the fuselage and nacelles and on any area which would catch the sun and reflect, turrets for instance.
The important thing is it should be subtle though. On a real aircraft the reflections would move around as the plane maneuvered and they won't when they are painted on a skin. So keep the edges of the reflections very blurry and reduce these layers' opacity down to a point where you wouldn't notice the reflections unless you knew they were there. I really do it to stop the skin looking like its just been painted grey all over.
