On iPhone SE (the original small one) the "I'm colorblind" checkbox is cut off below the bottom of the screen at 100% zoom, can't scroll to it or pinch-zoom out, so there's no way to know it's even there. I could see and use it when I set page zoom to 75%.

I thought I might have had the system text size set high, but it's at the 2nd-lowest.

Update: I love the colorblind mode! I was already going to suggest a game mode with more visually interesting emojis and you picked some great ones :)

I actually find it much more fun and aesthetic than the default mode. (At least on iOS, the emojis are very nicely designed. Iirc each Android brand has their own emoji design.)