Hey
I have the same problem with this keyboard, (mine has Alps SKCM White) and I will repaint the case. Just make sure to use the proper paint, which will be OK on ABS, check it out especially if you will use spray paint. Clean the case as much as you can before painting.
Another option is retrobrighting the case. You need a big container, where the case fits, you need 3% Hydrogen-peroxide solution, and plastic wrap, sunlight. Put the case in the container, sink it in the Hydrogen-peroxide, and cover the container with the transparent plastic wrap. (To ampify greenhouse effect in the container.) Leave it alone in the direct sunlight for at least 4-6 hours. It helps, if your solution is warm (30-40°C) and the container is transparent too. The case will be white again, as new.