They're looking for combat unit skins. Not training units, not prototypes, not captured planes. Those won't do.
When you find some color profile of a plane you like online, DO NOT rely only on that. Don't use it for color samples either. Use it as the inspiration to learn more about the plane. Most of those color profiles are very subjective, some are just low quality or "wrong" and others are just... well... badly drawn, badly scanned, badly converted, or some combination of the lot that makes them look horrifically different than the real plane did.
Just a word of warning. Research will cure that problem though. Research the plane and the time and the person/unit you're interested in skinning. Pictures are also most helpful (real pictures, not restored modern-era warbirds).