Recently I considered changing my sail covers and roller-reefing headsail UV shield from black to blue for aesthetic reasons. I was buying/converting my hank-on headsail to a new roller-reefing rig, and thought it would be a good time. As it turns out, a little research disclosed that dark colors are the best. Here in the Florida summer sun, we need all the protection we can get and dark covers not only last longer than white, but they will protect your sails better than twice as well. Ditto black thread on your covers. My mainsail, always under a black cover since new in 1978, is still in fairly good condition. Yes, it's been restitched and has a few patches, but it appears it will be useable for another couple of years. When a white cover signs off early, your sail won't be too far behind either. You'd think black would be bad because it absorbs so much heat. Actually, it blocks the UV rays best, and I've never had mildew on my sails (unlike a few neighbors with white or light covers)because the heat apparently dries them out quickly. So there it is. If your pocketbook can stand it, choose light colors. Otherwise it would be wise to consider black or at least some other dark color. Hold the material up to a bright sun and see if you can see any light filtering thru.