I'm not sure that is mottle on the rudder Wayne. Apart from dot-clash in the pic, due to its being a copy from a litho-printed copy, there's also some slight reticulation. A close inspection of the area where the kill bars are, shows an even colour which matches the remainder, and is a different tone to the base colour of the fuselage (presumably RLM76, possibly 65), and I think this is yellow. It's possible that a mottle has been applied to this, but doubtful, and again, I think this is reticulation in the image.
The yellow paint used for tactical markings up to this period was a distemper type paint, and painting over the top of this would cause the yellow to peel, crack, flake etc., unless another distemper paint was used - highly unlikely for a camouflage colour.
The cowling might have been yellow at one time, for instance immediately before the move from France, but again this would have been distemper, and would have been removed by washing it in petrol (gasoline). The apparent difference in density between the cowling and the rest of the fuselage might be, and probably is, due to the remainder weathering, where the cowling paint is relatively fresh, having been protected by the previous washable yellow coat.