During the major course of whitening or right after that the patient should be careful with staining foods or drinks. Brown beans never seemed to be part of this risky group of foods. You should ask the dentist who prescribed you the bleaching.
Sorry to hear you stained your teeth soon after whitening them.Everything that can stain a white Tee shirt can stain teeth. This goes for whitened teeth as well. You should have been advised to received a maintenance ( touch up) kit so that you can always keep then as white as you go them :) .
Anything that can stain a white tablecloth can stain teeth.
Yes the brown beans can stain teeth, especially during the 24-hours immediately following the bleaching. It is in that time period that the teeth are MOST susceptible to staining, before they get re-hydrated.