Yeah engine bay temps were mentioned on another forum, as well as weather.
Also wouldn't the black one get heat soak quicker as it would absorb heat quicker than a silver one?
You could go into so many levels on this, would the type off primer make a difference? It's acting as an insulating layer.
Thickness of paint, type of paint, alternative coatings? Ie anodised, powder coated.
Maybe we need the real Mythbusters to invesitage?