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.
I think chemistry ≠ physics.
While you're correct that the black object will both absorb and radiate heat faster, you are incorrect that they will reach the same equilibrium temperature under constant illumination by sunlight. You're not considering reflection, and the 'white' paint will reflect more of the incoming solar radiation than the black paint, meaning the equilibrium temperature (combination of the effects of absorption/radiation and reflection) will be higher for the black lens.
Try a simple empirical test - go to a decent-sized parking lot on a sunny day, find a black car and a white car parked next to each other that have been there for a while, and put one hand on the hood (or perhaps bonnet in your case?) of each. Your statements suggest that they will be the same temperature, but you'll find that's not the case. Don't leave your hand on the black one too long...