I am not sure of why certain colors do that, but I suspect it was just a paint formulation issue, that only affected certain pigments.
A case in point: Many people used to come into my shop and begin to tell me of their green Honda CB750. The only problem was, that Honda never made a green CB750. They were originally brown, but when the finish was exposed to UV for a couple of years, it would turn green.
POR 15 also has a problem similar to this. It isn't UV resistant, and when exposed to the sun for a year or so, it will flatten out, then slowly fade through to expose the metal underneath.
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