This assumes that Sasha's only concern with food consumption is nutrition alone, where other aspects of discipline and adherence are likely to also be important.
Asking for red, green and blue but specifically no brown M&Ms doesn't mean that you think brown M&Ms are fundamentally different in any way: there was another reason why Van Halen did that.
The bell peppers in particular sound like believable pseudoscience. nightshades are poisonous and if one is looking to avoid bloating, perhaps it’s a legit concern.
Something about their approach appears to be working.
It strikes me that creating pseudo-science rules might remind you and help you to also follow the really useful rules, through dedicated practice and having multiple fall-backs.
For example, the rule "Eat green bell peppers not red bell peppers" reminds you to always be consciously aware that bell peppers and then vegetables in general are an extremely important part of your nutrition. If you don't have superstitions about bell peppers, maybe you just aren't thinking about them often enough.
Even if that wasn't the case though, what do you expect to be gained by enforcing that discipline?
You have a finite amount of variables you can control, and wasting time thinking about details that don't matter is a massive drain. When training for a show, you should be focussing your effort on the things that make the biggest difference: overall food intake, macros, exercise, resistance training, meal timing, micros, etc.