For the 1966 Topps, there is 'bright' back differences in other series as well, due to the different card stock (grey vs white). At least for the first series, if not others (I don't have the energy to verify right now). I separated these card fronts a couple weeks back to prove the color difference on the back isn't from fading.
They get no love as white vs grey/cream variations. *sigh*
