So, there's this test. I've seen it before, and I've tried to do it before, and it's supposed to be able to give you the ability to see a color that doesn't exist anywhere in nature, that humans normally can't perceive. Here is the test:
Previous tries have revealed only the gradient to me, and I had worried that my brain might just not be able to process the existence of that third color. Maybe I still can't, and I'm just fooling myself. (It's probably a character flaw, but I find I get very envious of the existence of people who, though biological quirks, are capable of perceiving more reality than me.)
I don't know how to describe it, which is probably a step in the right direction, and I could imagine a bunch of things that seemed like they might sort of be the right color, but really aren't. It seemed sort of like a richer, darker beige. I can't hold it, though. It only shows up in glimpses, like what it would feel like to look straight at something that still felt like it was in the corner of your eye. Maybe this is what perception filters feel like.
I'm in a dark room, now, and I've got the test image full-screened. I think that might have helped. Try it out, and let me know how it goes in comments.