The relationship between colour and temperature of bodies receiving radiation is not as straightforward as 99% of postings on the net say. Although black absorbs heat better than white, it correspondingly radiates heat better. The inescapable consequence is that black and white bodies reach the same temperature when they are in sunlight but black gets there faster. Conversely, the black body will cool down faster when taken out of sunlight. So, the Nikon lens heats up faster than the Canon L, but both eventually reach the same temperature and the Nikon cools down faster.
While not as straight forward as some postings would indicate, the basic fact that a typical flat black object will reach a higher temperature in the sun that a typical white painted object is correct. The black traps more of the suns energy, the white reflects it. That is why things like propane tanks are painted white. You can verify this yourself on a sunny day. Put one hand on some ashphalt surface and the other on same grass alongside it. The asphalt will be much hotter than the grass, guaranteed.