Here is a shot of our dark green cover.
It does seem to get the water warmer - but that is good for us - if we need it to cool of during a hot spell, we can just open the pool at night and/or run the pump at night instead of during the day. Right now we are sitting at 83° in northern California - with no heater...pool water this year has been in the upper 70's-80's for a month or so...with some cold weather in that time frame.
edit:
Our original choices were between gray (way too much gray for our yard), dark blue, or this dark green. We thought about blue since it would sort of look like pool water - however green was our choice since it matches our house better.