I noticed the same problem Vraciu refers to in one of the skins I'm working on, and did a little experimenting to see if I could determine the cause.
The problem is that black or very dark areas show up correctly in direct sunlight but in shadow show up as a dark grey with a little bit of shine to it. Here's an example using the P-38 nose gear:

Interestingly, initially I also had this effect on the sunlit side, but it went away when I changed the _E map to a pure black bitmap (x'000000'). It went away on the sunlit side only, however.
I continued to change the support files to pure black one by one and noticed any change - but nothing else made a difference. The screenshot above is made with _E, _S, and _P maps all pure black, and a completely blank _N map.
I'm wondering now if this is a limitation of the graphic engine? I don't seem able to affect how the shadow side is shown.
The screenshots were taken on CraterMA, A55, 10:00 am game time, at the SW runway spawn.
EDIT: This effect is not present in the default P-38J skin. I tried removing all my support files (_E, _S, _P, _N maps) and copied the _S and _N maps from the default skin folder - however my skin still has this effect.