Oh, I believe you. Just pissed off at my local Costco.
Yes, it's true that color does not exist in the physical universe: it exists only in the brain. But this doesn't mean that one person can see blue internally as yellow and that another could see it as red. The reactions and recognition of colors is neurological in nature. Once it occurs there isn't yet another evaluation that takes place. Blue looks to me as blue in precisely the same way it does to you, by definition.