Still, it can be quite difficult to see beyond the feeling that red is reality. Apparently, though, that feeling of something being real is, like the color red, something produced in the brain, and not something produced by a reality apart from the brain.
I like your post a great deal, and it makes many arguments I am in my upcoming book (whenever that takes shape). However let's put a finer point on this. What you say about the difficulty in seeing beyond the feeling that red is the things reality is very true. Red is a metaphor, a way for us to hold experience with our minds. But what happens in time as we learn these word-signs is that the metaphors collapse into descriptors of reality. They become dead metaphors. We "concretize" the metaphors, make them the facts of the thing itself. And so we fuse together the experience of reality with these descriptors, or worse, which happens in memory of experience, we shortcircuit reality and replace experience itself with the experience of words, living inside a world of linguistic signs. We experience the signs and call that reality.
When it comes to mystical experience, the goal and purpose of it is to move beyond words into as best as possible an unmediated reality, into experience itself. That said however, interpretation of it does in fact happen after the fact, or even right alongside it during the experience itself if it is within what are called subtle-state experiences; encounters with the divine in form, such as light, deities, devas, and so forth. In these cases the nature of symbols takes on a different characteristic, which truly is moving back into the realm of actual metaphors. They are simply open-ended, rather than descriptors which are the husks of a dead metaphor. If one consciously understands that the manner in which the mind interprets, or rather translates the experience itself is not 'factual', not the actuality of the thing itself but the mind's symbolic representation of it (this is a metaphor), then it does not get confused that what it encounters is the final or absolute truth of it.
The feeling of the color red being real is the illusion of the mind, yes, imagining that there is an actual thing called "red" laying around on there in nature, just like imagining God is laying around out there. But it is in fact true they are experiencing something. There is also the fact that the words that people use convey something actually meaningful about the experience, such as 'oneness'
The question one has about the mystical experience (i.e. the experience involving a sense of the oneness of all things, etc.) is whether and in what way it might be like the color red. That is, it seems wholly possible that the sense of oneness has the same relationship to reality as the color red has to reality. It might be that it is not a property of reality, but rather a response of the brain to some reality that "oneness" does actually represents. Is "oneness" something that is a factual thing laying around out there, in the sense of objective reality, or is it the highest experience of the human being in response to objective reality?
I'd say both actually, but that will get a lot of detailed and deeper than I want to go at the moment. We are experiencing ourselves as human, both as subject and object, both of the world, and the world itself. There is an objective truth to this as well as a subjective response as a human. But we have to be careful not let our metaphysical language we use to talk about it become a description of reality itself. Reality has to remain open, even while we close off little bits of it in order to talk about it with one another.