Suppose that I see red the way you see green. Then, when I see a stop sign or a fire extinguisher, I'm seeing the color that you would call "green". But of course, I have been told since birth that fire extinguishers, stop signs, etc. are red, so I would naturally call that color red. And in the same way, if I see green the way you see red, when I look at grass I would be perceiving the same color that you see when you look at a stop sign. But I would call it green, because everyone knows grass is green, what else would I call it?
There would be no disagreement between us about what objects are red, no way to tell that we see them differently, and no real-world consequences that would force to rewire any neurons, because our ways of perceiving the world would both be coherent and equally valid.
By that sense, if qualia have no discernible impact on any part of the system, in what sense do they exist?
One can take this thought further. Red vs green light have real physical properties that affect one's subjective experience of them. For example different colors' physical properties change the way that they are perceived in low light conditions, the way they are perceived when placed next to other colors, etc. So at the end of the day even if hypothetically one person's qualia for red is swapped for green, you end up with a green that acts an awful lot like a red for all intents and purposes in the brain.
Edit: So my personal hunch is that qualia can't be meaningfully said to exist.
> Suppose that I see red the way you see green. Then, when I see a stop sign or a fire extinguisher, I'm seeing the color that you would call "green".
Are you sure it is possible to pluck the color I'm seeing out of my head, and the color you're seeing out of your head, and compare them side-by-side, like we could compare two different colors of paint on a piece of paper?
I'm not so sure about that. It seems to depend a lot on what phenomenon "how you see a color" actually refers to.
If "how you see a color" refers to the neural patterns excited by light of the relevant wavelengths, and you and I have the same neural patterns, then we see the color the same way. There is no metaphysical layer on top that can make one of those experiences green and the other red.
There would be no disagreement between us about what objects are red, no way to tell that we see them differently, and no real-world consequences that would force to rewire any neurons, because our ways of perceiving the world would both be coherent and equally valid.