Yeah, I was less concerned about the usability of the app itself (I assumed there are other challenges available and they're configurable) but it's a bad first impression, especially when it's the only challenge shown and the screenshot is repeated multiple times throughout the landing page (which suggests it is the only challenge currently available).

If you're going with a "color blind" palette, just make sure you don't accidentally make it worse. I've seen many people mistakenly avoid e.g. red/green completely and end up with hues that are even less distinguishable.

A good low-tech approximation is to just check your palette in gray scale and make sure e.g. red/green or yellow/blue don't appear too similar. It's likely not necessary that a user knows which color each color is supposed to be but they will likely need to be able to distinguish them at a glance. Also make sure when filling in a shape with color (such as letters) that the shapes are big enough (i.e. bold enough) -- colors are easier to distinguish when they cover a larger surface (more photons, better vision).