My kitten had a white chin until 2-3 days ago when it started turning brown-ish. There are no bumps or red spots, so I don't think it's "acne." Could the discoloration be due to the clavamox & lysine he's currently on for an upper respiratory infection?
Without seeing your cat in person, I'm not sure whether the discoloration is occurring in your kitten's fur/hair or his skin. You also haven't mentioned how old your kitten is. If your kitten is quite young, he may actually be changing color, as many young kittens coats change color as they get older.
Otherwise, yes, certainly any cat medication that is being given by mouth can soil the fur around your kitten's mouth and chin and begin to look dirty. Some kittens are also not great about cleaning themselves after eating, and may be worse about doing so when they aren't feeling well.
You could try taking just a warm, wet cloth and cleaning your kitten's chin. If the cloth becomes soiled or colored, cleaning your kitten's chin twice daily might help.
All the best,