get a 12 mil blue/ silver. the thinner ones will only last a season, the thicker ones don't last that much longer than the 12 mill to justify the extra price. Blue/silver is usually the least expensive and works as well as any color. To use a solar cover correctly you cover the pool at night to stop evaporation, which is how and when the most heat is lost, and then uncover it during the day so the sun can warm the water. some covers claim that they will collect heat and transer it to the water but it really dosn't work that way.
Also, you don't want to leave the cover on continueously or you will get a buildup of chloramines (combined chlorine) in the water. If you want your cover to last as long as possible do not leave it in the sun when it is off the pool. Either keep it in the shade or cover it with an old WHITE sheet or WHITE plastic, such as kitchen garbage bags!