Why is is okay for black people to make fun of people based on their appearance but its wrong to make dun of them on their skin color?

I was walking through my apartment complex and a group of black people started making fun of my disability and the way i walked. To make them stop I said the n word and they all of a sudden were shocked and wanted to beat me up. Why is it okay for them but not okay for any other race to do the same thing.