As a colorblind person myself, this would be very helpful. I am not monochromatic, I have Deuteranopia (think of it like RGB, but without the G).
I've found that I don't really associate things with colors. For example, for the longest time I didn't know that the Hulk was green. I just thought he got really big and angry, lol. Then one day, someone mentioned that the Hulk was green, and that's when I realized it. Now that I know he's green I can see that if I'm looking for it, but my brain isn't wired to associate color with things, I would have never noticed he was green if someone didn't bring it up. Another example is with a stop sign. When I see a stop sign, the first thing that jumps out to me is that its a hexagon, not that its red. In the same way, I find Taiko to be hard to read because both Don and Kat are the same shape.
I would love to be able to change the shapes via skinning, I think it would be much easier for me and other colorblind people to read. However I don't want it to give an unfair advantage. I think it raises the question of "Is part of Taiko's difficulty in learning to read the colors?" I would love to have some top players try it out, and see if they think its fair.
Also, for non-colorblind people, does reading Don and Kat by color come just as naturally as other reading in rhythm games? Or was learning to read the colors part of the process?
For monochromatic people, I imagine that Don and Kat look pretty much identical, making Taiko unplayable. I think what would be completely fair, would be to add a Taiko colorblind mode. This would change the colors from red and blue, to maybe a brighter red and a darker blue, or even yellow and dark blue. Anything that would make it playable in grayscale would be a godsend for people with monochromatic colorblindness.