I’ve used buttercream on styrofaom. However, I wrapped the styrofoam in plastic wrap first, then I iced it with buttercream. If you use hot pink icing in any of the decorations it will most likely fade, and black may bleed.
I’ve done this for seasonal displays. After the season is over I take the icing off and peel the plastic wrap off. If I want to use the styrofoam for another display I just rewrap it in plastic and decorate it again.