This discussion is interesting in terms of the visible portion of the light spectrum, but folks, that is only a small part of what's involved in solar heating. Visible light heats up things that absorb its energy, things that look black or at least dark and they in turn heat the air or water around them. Air and water look clear because they absorb almost no visible light energy.
But HEAT radiates in the infrared (IR) portion of the spectrum, which we cannot see naked eye. Water is 'black' in a key portion of the near-IR, which is to say that it is a great absorber of solar heat. (At ~3300 1/cm) Visible light goes down and comes back through > 20 feet of clear water, but IR heat barely gets down 2 feet. So get a solar cover that looks nice and lasts long, and run your pump to circulate heat away from the surface. If you want to convert vis light to heat, paint your pool bottom a nice dark blue or buy a dark liner.