The thing about green uranium, and green radioactive waste, well, lets just say that popular culture gets a TON of stuff totally wrong. Hence, we kinda have to keep with it, rather than making uranium yellow.
yellow CAKE is yellow, it is simply a dust which has a bunch of uranium precipitates in it(correct me if my chemistry is wrong). If anything, uranium dust is grey, because the metal itself is grey. Only uranite is green and that is from where the "green uranium" rose