I use satin ribbon on my buttercream cakes quite often. The grease from the buttercream does darken the ribbon. I do inform the brides about this when trying to match their colors as close as possible. I usually try and go a shade lighter to allow for the color darkening when choosing a color. I’m not sure how to prevent this, but have never had a problem or complaint when doing it directly on the cake. I use a crusting buttercream, so when it comes time to cut, the ribbon just comes right off.