My experience with solar covers is that it is much more about conserving the heat you have than heating. Keeping the cover on at night, especially if it's cool, dry and breezy will help slow heat loss due to conduction and evaporation (which may be quite high). It may even slow loss due to radiation (silver probably wil help here) - it would also probably slow the radiant heat the pool gets from the sun an equal amount.

What I'm trying to say is it may be best to think of the solar cover as a more of a blanket than a heater.

I have blue because I like the color.