Is my work colleague being racist to me, and what come back should I use?

The comeback in the US would be to file a complaint (harment or discrimination).... That's definitely racist here. I dunno, coming up with a comeback seems a little short sighted, sort of like George Costanza going through a whole Seinfeld episode trying to come up with one. When he finally did and used it, the other person immediately came up with another comeback and he was left speechless again.