I'm in Canada where heat retention by solar covers is critical. My rule of thumb is to always use it at night, and to take it off in the day only when the sun is shining and there is little wind (or when I'm using it!). This seems to be the best combination. I do find that when the cover is on and the pump is off on a sunny day that the top 4" of water is very warm, so solar gain during the day is definitely there, but I do agree that the lack of cooling by evaporation (my cover claims up to 95% less evap.) at night is the more important factor, particularly in the shoulder months. With the cover and the pump both off during the day, the same amount of daytime heating may happen, but is more evenly distributed throughout the pool depth. I am not sure how you would determine this.
Previous discussions here suggest that the clear covers may be better than the dark blue ones for heating the pool during the day. Seems logical to me, but the covers available in this area at a reasonable price are all dark blue, so I haven't been able to test.