here's what i would do from (i don't have cbars myself, but i would imagine that the materials are the same as normal goggles)
first, get a paper towel/napkin or something and get it wet. Then, rub on the frame. most likely it'll come off
if that doesn't work, get the pouch/baggie that they come in (cloth), and attempt to get it off with that.
if taht doesn't work, then i would have to say get a white marker and go to town, hopefully you wont have to though...