Why do some users want color blind support given iOS already has it?

Even if the OS-wide colorblind mode is well-implemented, it's a one-size-fits-all approach that doesn't take into account the specific functionality and design choices of a particular app, game, or website, or the differing levels of colorblindness even among a specific type like protanopia.

My partner is partly colorblind, but the default colorblind modes in a lot of games don't help her. The color palettes chosen by the game designers are still hard to distinguish even with these modes enabled -- even for stuff as simple and crucial as "my team" versus "enemy team". She says the best games are ones that let her customize the color palette to her specific needs. This would also help users who aren't colorblind but find that certain high-contrast palettes are easier to see -- or easier on the eyes, a la dark mode.

In short, the built-in modes are better than nothing, but they can't make up for poor design choices.