John-
It doesn't damage the decals to paint over them. Along with the paint, the markings on a real helo will fade over time. In the pic below note how faded the rescue label is on the door compared to the relatively new logo and aircraft number.
To replicate this, just airbrush over the decals with a lightened shade of the paint the decal is on. I thin the paint more than usual, and turn the pressure up a little, to get the finest atomization (smallest paint particle) so you can't see individual paint specks on the decals. Hold the airbrush a little farther from the model than normal, and lightly spray over them. Go slow, and check your progress often, you can't undo it. Compare the stars and intake warnings before
and after overspraying. I added the skull logo and a/c number after overspraying so they don't look faded.
I think it works pretty well. Give it a try, but maybe try it on a junk model first, it's easy to overdo it. I hope this helps.
Phil