I was told that color would not make much difference in the heating effect of a black plastic flower pot because the sides of the pot are nearly vertical. My understanding is that holding other things constant, a lighter color would be more effective in reducing heat buildup than a darker color. The question sounds a bit wonky, I know, it came up in a gardening group run by university extension agents, normally pretty well-educated people, so I am reluctant to butt heads without a bit more information.
I assume the discussion is about heating of the pot due to sun. The reflectance of incoming sunlight is considerably greater for a white surface than a black surface. The incoming solar radiation will change during the course of the day. I would argue that the lighter colored flower pot will be cooler. Such is the basis for cool roofs (Cool Roof Rating Council). I suspect the extension agents are saying the difference might not be large enough to be of concern. Temperature increase will depend a lot on the water content of the soil in the pot. If there is a lot of water in the soil, then evaporation will help keep the contents cool. Hope this is helpful.