Was thinking about your comment for the past few days. And a couple days ago during a 20 plus mile exercise ride, noticed several other riders out for exercise and I thought about your comment on helmets when I was looking at other riders. I concluded that when looking at them at a distance, color of helmet almost did not matter. The amount of surface area that I saw from the front or behind them of their helmet was about the same as the surface area of one short sleeve on a jersey. Jersey (or jacket) was the obvious place to put something to be seen, helmets not so much.
Over the years I have discarded three helmets from age, probably mostly from UV damage. At this time have four helmets: high vis color, white, red and black. I did not need a fourth helmet last summer, but a bike shop in Halifax NS (Cyclesmith) was extremely helpful, so I felt obliged to spend some money there, and bought the high vis helmet plus a jersey.
Years ago I recall hearing that the reason that tires are almost always black is that the carbon black is added to tire compounds to make them more UV resistant to make them last longer. When I go out on a long ride on a sunny day, It has been automatic for me to grab the black helmet as I assumed that helmet had the best UV protection.
I was now curious about whether or not black tires were more UV resistant, so a quick google search yielded a few links, and I opened this link that confirmed the carbon black was good for UV protection, although it said it also served other purposes:
https://www.mentalfloss.com/article/...ires-are-black
So, I will continue to preferentially grab my black helmet on long sunny rides. Will a black helmet last longer because of better UV protection? I have no clue, but unless I hear differently, I will assume it does.