What he said
Add noise to the colour layer but also, you can de-saturate the colour ie. shift it towards gray. I don't know how you do it in PS but in PSP it's SHFT+H (or CTL+H maybe) and you should get a little window with 3 sliders ... desaturation will be there. Use a fistfull cos the specular lighting in IL-2 is quite powerful.
The other thing is, avoid abosulte colours ie. absolute white. Use a very light grey, check it in IL-2 and the lighting will make it look white

well sometimes