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!
xx
If you look at my pix, Megan's wedding cake, I used black satin ribbon (per the bride's request) and adhered it with piping gel with a seam in the back. I did it just before we transported it because I gave each tier a "Crisco bath" and I didn't want the ribbon soaking up the oils. It didn't and the look worked well.
to attach ribbon to fondant i just wet the ribbon first (not too wet) and put it around the cake, when the ribbon dries it should be stuck to the cake with out the marks that you sometimes get with royal icing.
No water marks? Sounds like the way to go.
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