deskthority

Does anybody know if something similar exists in Germany or how to get some?

You should be able to find some on ebay.

Any textile dyes suitable for synthetic fabrics should do the trick. Look for dyes that can be used to colour buttons.
Actually, I'm thinking of dying a few keys of a model M.
Dylon Universal looks to me like a good candidate:
http://www.patin-a.de/pi/Dylon-Universal-Color.html

I didn't know you could get dye for buttons as well (just never thought about it).

I've dyed clothes in the past, and I've been caught out with the thread (that holds the cloth pieces together) - as it might be a different material and often doesn't change colour. Now I know what to look for.

I bought RIT dye from some US-where online, I think I found the place by searching for rit dye on amazon. Shipping wasn't a killer either. Haven't had time to test it yet though...

Powder RIT should be better than Liquid RIT. Make sure that you get the salt also.