Have you ever wondered why some lenses are white while some are black?
Well the reason for this is that lenses contain glass elements in. In high temperature glass expands. The expansion is very small and insignificant with compact lenses and so is never a problem. But very large lenses have large elements of glass, which can mean any expansion through heat can bring a lens close to the limits of its design capabilities.
But making the lens barrel white, helps to control this expansion through reflecting sunlight. This helps keep the lens cooler.
Its usually the telephoto lenses that are white. for example EF 200mm F/1.8L