I have no idea if someone else has made this already but here’s my take on turning your WaniKani experience down a notch into the World of Grey.
This is my first attempt at throwing a user script together so… advice welcome 
Update v0.2
UserScript URL (v0.2):
It looks awesome. 
Also you could create a greysclaed image for favicon, upload it to imgur and dynamically insert it in page header tag.
But I would suggest that you don’t remove colors. These colors are not just for making site visually pleasing, instead they play vital roles for distinguishing between radical, kanji and vocab.
More importantly since kanji usually teach On’yomi and vocab version teaches Kun’Yomi, Colors can help you remember which reading is On’yomi and which is Kun’yomi. It is crucial to remember the difference. But I guess you know all this already 
Even though grey scaled color are still distinguishable, a better approach would be to use color which are warmer to eye and are different from each other.
Uh I need help im new
what kind of help?
Removing colours is pretty much the whole point of the script! 
What I have just done though is make it so that kanji questions are visibly different by adding （漢字）after them.
