Just needs to block light if you're in the camp of high intensity light damaging roots. Maximizing root health and confusion over stoned reading of hydro setups more than likely leading to these masturbatory discords.
The reason some people swear on various colors is the light penetration. Lots of the thin walled white buckets and even the grey ones allow for more than the traditional black pigment.
With soil its almost pointless to argue this small amount in most cases. Hydro setups are where this potentially has more of a concern if trying to control certain secondary growths that can be encouraged by light.
Some people of course are trying to squeeze lumens. The argument over painting white, using white, etc to gain reflected light may have merit in smaller cabs/rooms. Very subjective needs.
Best soil pots personally are smart pots or similar. That's just my opinion on the matter. Color there could be an issue if your soil cooks (think extreme ambient temps above ground like desert growing). Never bothered to test soil temps to vet this idea of heat transfer from the surface inward. Evaporation and shade from the plants takes care of most heat issues around the main root mass regardless ...
Just more voo-doo than anything most of the time. Next discussion can be on the color of mono-filament line for fishing.
