Black and White of lenses
we are often fascinated by the long white (actually a light grey or beige) lenses from Canon. These white lenses are all large super-telephoto lenses from Canon. There is a good reason behind this and not just cosmetics.
Various glass elements are included inside the lens barrel. These expand with heat. This is not usually a problem with compact lenses as the amount of expansion is quite small. in compact lenses. As large lenses contain large elements the expansion in these can bring a lens close to the limits of its design tolerances. A white surface reflects sunlight, helping to keep the lens cooler.
Also, you will notice that the manual focusing action of some black lenses allows focusing past its infinity setting (also known as over-focusing). This is mainly to allow for expansion. If the lens is used in hot conditions, infinity focusing will be closer to the end of the travel.