The white cap with red character makes sense. It's a reference to hanko, or Japanese style stamp seals. Artists make their own, and put the seal on their works. The name of this set is "Sumi", so having the kanji for sumi (墨) be written in red on white makes perfect sense, to reference hanko stamps. If you can make the white similar in color to the mod keys it would be best, but I like it a lot as-is.
Edit:
That being said, stamps that create red background with white text are also used commonly in art, so that would make sense as well. I guess my only argument is that it shouldn't involve black, even if the word means black ink.