I have a black one, blue on the bottom. Blocks a lot more of the UV rays. I have a heater, so I'm using to maintain heat at night, and minimize FC loss during the day. I can go days without adding chlorine if the cover is on and I'm not using pool.
However, that being said, Its my perception it breaks down faster than typical. It's 12 mil but the bubbles are breaking quicker than with other covers I've used. I'm not sure what the cause is, really. It's NOT my water chemistry, LOL I suspect it maybe from sitting in the sun when the pool is used. I'm not diligent about shading it when its sitting on the deck.
It's about the same cost as regular covers so I'm not worried about that. I do like being able to skip a day or 2 of chlorine worries.