Opinion on replacing Minolta AF 100-400 APO.

The heat absorbed from sunlight is going to be based on how much the color reflects or absorbs IR.  That's not something you can tell reliably from the color you see. My one data point in Death Valley seemed to indicate that the particular white paint Canon uses does absorb a fair amount of IR.

I doubt the focusing motor generates significant heat no matter the color. It generally only runs for brief periods.  If it generated lots of heat it would also have to eat lots of battery to get the energy to produce the heat, and while the 70-400G does go through more battery than that of smaller lenses it's still not all that much.