In truth, they're probably all correct. I don't know if that helps you or not. Vehicles, just like people, each age a little differently. Unless your model depicts something fresh from the factory, the paint tone will begin to change as soon as it is exposed to sunlight. Add in, the place it serves, the duration of service, maintenance intervals, or lack of maintenance intervals and you get the idea. So long as you're not going for a specific vehicle at a specific moment in history in history, whatever you have is probably the correct color at some point in time. Even a broken clock is correct twice daily. My advice is to use the paint which you like the best.
EDIT: I just followed Rob's link above. Yeah, what he said.