For any vehicle that is ordinarily left outdoors (not hangered or garaged), the sun will change the color in a couple of weeks. So I only worry about color accuracy if I am depicting a new car, or a military subject on the day it is rolled out of factory.
However, since I love to weather, that means I don't ordinarily worry about color accuracy on anything military. But that may change. Newer paints on military subjects are becoming much more resistant to UV.