Some models come with open cowl flaps… like the He-177 and He-219… the question is: Which color must I use for painting the inner surfaces of them?.. maybe an interior color or burnt metal?..
Depends. Some cowl interiors were left unpainted, others had an interior primer. Either way, showing heat effects from the exhaust pipes would be an excellant way to go.
Actually, most of the discoloring found on the inside of cowls and cowl flaps come from oil and exhaust leaks. I was taught many years ago that if aluminum (which most cowls and cowl flaps are made of) gets hot enough to discolor it, then it has been severely weakened which means, A. you need to replace it and B. you need to find out why it’s getting so hot and correct it. (a quick indication of a problem is the paint on the outside is charred and bubbleing)