Solar pool covers have been studied by the US Department of Energy and are considered to be the best way to heat your pool. They don't work as much by getting hot and heating the water as they do by preventing evaporative heat loss over a large surface area. Here's a good read on the subject: http://energy.gov/energysaver/articl...ng-pool-covers

They do indicate that a completely transparent cover will work best in that article.

My personal experience is only with a blue one. It was a big pain in the butt and I stopped using a cover after one season.