For some of the easier ones, I was surprised by how aggressively my brain was making what seemed like "close" colors stand out. Like it was mentally "highlighting" the odd one out. I thought that was cool, but eventually the later ones were hard enough that the effect went away, and I was much less confident in my choices, and I got 3 mistakes, all in the last five rounds.

The way your brain manipulates your vision 24/7, with no way to get around that, is truly crazy to me. There's all sorts of effects in your visual system like edge detection and certain types of stimulus suppression that it's crazy we even feel like reality is coherent.