I dabbled in painting some fantasy figures about a year ago, not for gaming purposes, but just because they looked cool. I've always liked dragons, and there's a company called Reaper Miniatures that does some very nice figures. Anyway...I digress.
Games Workshop produces both the kits and the paint line of Citadel, so obviously they're going to reference their own products. To my knowledge, there is no obvious cross-reference that says that Scorched Brown is the same shade as RAF Dark Earth, etc.
The only real way to accomplish this is to create a paint chip chart using your Citadel paints, and match them against other known colors from other paint lines.
This is helpful if you want to use Citadel paints on historical subjects like a Sherman tank. If you're working in the other direction, as RemcoGrob said, it really makes no difference what colors you use since there's no historical standard to compare them to. Do what you think looks best. I'd break out the DVD set and compare the colors of your paints to the colors you see in the film and just have fun.
Hope that helps!
Jeff