Eww. The ultra thin base color overcoat works wonders. I just did it for the first time, but with NMF that's not an option. If already on the model I'd try some light colored pastels like white, buff, grey, or similar. I've used buff colored before and it looked good. You can clean these off if you don't like them.
Another thing I've heard people do is spraying a dilute coat of paint in whatever color you like onto the decal sheet. If you're putting them on a grey plane then a light coat of grey would be appropriate. As for what to use for NMF I'd experiment on scrap decals or something. People use buff colored paint to fade armor...it'd probably be the first to try. Please don't just go spraying the decals you're going to use though. I want you to be sure you're going to like the result.