you dont have to worry about this with fondant. just secure it at the back of the cake - royal, bc, double sided tape, glue dots, a pin, whatever.
on a bc cake, it may not absorb anything anyway. but you can just grease the whole ribbon, so that any colour change is consistant. some people iron wax paper over the ribbon to provide protection. there are lots of posts on how to use ribbon on bc cakes, they have more tips too, i just cant remember them!