This is an interesting concept I've never heard before. I've seen the spf mentioned on a shirt, but as Pika mentioned, I've never been sunburned through a shirt before. As far as colors, darker colors will absorb more radiative energy from the sun. That means you will get hotter. For that reason, if I was looking for a sun protection shirt, I would get a lighter color. It is interesting that it sounds like you would go the opposite route.