Years ago, I did some thermal testing of white vs black helmets under simulated sunlight.
With air moving at 5mph, the white helmet was 1.5ºF above ambient.
Under the same conditions, the black helmet was 2.5ºF above ambient.
Conclusion: As long as the air is moving even slightly, the white-black temperature difference is not significant.
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Ride, Rest, Repeat. ROUVY:
terrymorse