But we are talking about two similar built lenses, light concrete might be hotter than grass, but it is a lot cooler than black concrete or tarmac in the same sun.
I understood, and I vaguely remember this being from a Canon Lenswork book a long time ago, the white lens colour was introduced solely because of the specific contraction and expansion characteristics of the artificial fluorite elements. Indeed wasn't the first artificial fluorite element in the Canon FL300mm f5.6? It was black with a green ring (pre DO and FD!) and came out in the early '70's. When they did the same thing with the black FD300 f2.8 (that also had a green ring) they did have "issues" with focus, that is also the reason all Canon teles used to focus past infinity, so on the update of the 300 f2.8 they went white specifically because of the fluorite elements thermal characteristics and all the subsequent teles did.
I am sure it has since become a marketing tool, but there was a real reason for the introduction.