Y'know, after years of using many black Nikon lenses with a gold stripe, even large long ones, it really does still feel odd to me to now have a couple of lenses which are grayish-white! First the Bazooka and now the 100-400mm.... Apparently, like Canon, Sony chooses to make its longer lenses that light color rather than sticking with black, but I am not sure why -- need to look that up. Perhaps something to do with dealing with the heat and the sun, since often long lenses are used for hours during sporting events as well as when out shooting wildlife?
ETA: a quick search answered my question and I was correct. Seems that heat causes expansion of glass elements, and a light surface reflects sunlight, which helps keep the lens cooler. This can prevent the lens from overheating, which could affect the internal optics and thus image sharpness. So, clearly Sony has taken note of Canon's strategy with regard to its longer, heavier lenses and done much the same.