I'm just not seeing what obvious reality it highlights, so either I'm particularly dense or it's not in fact obvious.
So, rephrasing: what reality is being highlighted by the "illusion" ?
Your prime number analogy suggests that it's in fact the "both colors are the same" assertion which is an illusion. The perceptual reality is that the pixels in these areas are discriminated as different colors. The illusion consists of looking at pixel with identical RGB values and thinking "Oh, these have the same position in colorspace, I expect my brain to perceive them as identical."
The reality suggested by the "illusion" is that this expectation doesn't hold in general, it's a stupid model. A smarter model would take more things into account before it predicted what our brain will perceive as identical colors.
But this is very much non-obvious...
I'm just not seeing what obvious reality it highlights, so either I'm particularly dense or it's not in fact obvious. [...] But this is very much non-obvious...
The post is keying off of Think Like Reality.
...Reality has been around since long before you showed up. Don't go calling it nasty names like "bizarre" or "incredible". The universe was propagating complex amplitudes through configuration space for ten billion years before life ever emerged on Earth. Quantum physics is not "weird". You are weird. You have the abs
Today I looked at the above illusion and thought, "Why do I keep thinking A and B are different colors? Obviously, something is wrong with how I am thinking about colors." I am being stupid when my I look at this illusion and I interpret the data in such a way to determine distinct colors. My expectations of reality and the information being transmitted and received are not lining up. If they were, the illusion wouldn't be an illusion.
The number 2 is prime; the number 6 is not. What about the number 1? Prime is defined as a natural number with exactly two divisors. 1 is an illusionary prime if you use a poor definition such as, "Prime is a number that is only divisible by itself and 1." Building on these bad assumptions could result in all sorts of weird results much like dividing by 0 can make it look like 2 = 1. What a tricky illusion!
An optical illusion is only bizarre if you are making a bad assumption about how your visual system is supposed to be working. It is a flaw in the Map, not the Territory. I should stop thinking that the visual system is reporting RGB style colors. It isn't. And, now that I know this, I am suddenly curious about what it is reporting. I have dropped a bad belief and am looking for a replacement. In this case, my visual system is distinguishing between something else entirely. Now that I have the right answer, this optical illusion should become as uninteresting as questioning whether 1 is prime. It should stop being weird, bizarre, and incredible. It merely highlights an obvious reality.
Addendum: This post was edited to fix a few problems and errors. If you are at all interested in more details behind the illusion presented here, there are a handful of excellent comments below.