"Your red may not necessarily be my red" is one of the most basic entry level concepts in philosophy. Like I remember discussing it with my friends when I was a tween and younger. Which makes it surprising how many times I needed to remind people on Tumblr that it was a thing.
Except now looking back on it, I don't think it's so much that they forgot an elementary philosophy concept so much as they were incentivized to not think about it, because not thinking about it gives them more latitude to lord it over those they consider inferior. And I don't think I was reminding them, so much as I was tearing apart the logic they used to exercise entitlement to the insides of other people's minds.