Technically, the greatest heat gain comes from a clear insulating cover because most of the sunlight is then absorbed by the water but the heat is largely retained due to preventing evaporation by the cover and with an insulating cover by preventing conduction/convection (though not radiation) of heat. As I note in the thread Water Absorption and Heating from Sunlight, around 60% of sunlight is absorbed in a white plaster pool (a dark bottom pool absorbs even more).

An opaque cover will prevent sunlight from entering the pool to heat it directly. The color of the cover then becomes important where a white or reflective cover will heat the water the least while a dark black cover (especially one that absorbs infrared as well as visible light) will heat the most, but the heat transfer to the pool water will be limited due to convection and radiation and the uneven heating of the pool water. The best result with a black cover occurs if the water is circulated under the cover to keep it cool (same principle as with solar water heating panels) and if the black is on the underside of an otherwise clear plastic cover.

I have a mostly opaque electric safety cover and when it was tan in color it did not have any significant heating but now that it is darker blue it heats the pool a little bit. No where near what a clear insulating solar cover would do, but the difference between a lighter and darker cover is still noticeable.

Most heat loss from pools comes from evaporation. The next largest loss comes from radiation (for outdoor pools, not indoor) and the lowest loss comes from convection though if there is wind then convection losses can be greater than radiation losses, but losses from evaporation are still the largest by far. So any water vapor barrier cover will cut down most of the heat loss by eliminating evaporation. An insulating cover then additionally reduces losses from convection. A completely opaque and well insulated cover will retain the most heat, but it will also block sunlight from heating the pool directly. So indoors that would be best. Outdoors, a clear insulating cover is better as it will let sunlight add heat to the pool.

See this document for the technical methodology used by the EPA for Energy StarŪ performance ratings for swimming pools.