In the case of headlamps, 99% are now made from polycarbonate (the front lens/glass anyway). This polymer is unstable to UV so must have UV protection. Paints (the pigments in them that is) may or may not be sensitive to UV, so I'd expect the sensitive colours to have paint/laquer formulations that will protect the pigment. It may well be that all finishing laquers are UV stabilised whatever colours they cover, it would be logical. I'm sure there's lots of info on this on the Internet. Wikipedia will have it as well.
TC
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