Using Rice/Wafer Paper

I've used rice paper/potato starch sheets for a variety of things. You could trace out your design onto the paper with a food coloring pen, cut it out, then color it in, either with food coloring pens or brush on some gel cake color, let this dry, then paint a VERY thin layer of clear piping gel over it and let dry out. Now you can lay this right on a frosted cake (buttercream or decorator's icing for example), the rice paper will eventually desolve and leave your colored design. You could then pipe over this. This technique works wonderfully well when making butterfly wings, instead of attaching flat on the cake, attach wings to a royal icing piped bug body and you have an instant butterfly.

If you are wanting to pipe decorator's or buttercream directly on the rice paper, I wouldn't recommend it. It has been my experience that the icing desolves the paper. If you want the paper effect sitting on top of the cake (little sheets of music for example) let the icing crust over fairly well before putting the paper on and display/serve almost immediately. If you are using a large piece of rice paper ( a hand writting love letter on a Valentine's cake for example) then place the paper on sugar cubes to elevate it above the frosting and then you can pipe some frosting on the edge to hide the gap. (yes the paper will start to disolve on the edge in contact with the frosting but it will still stay in place and not dissolve beyond the edge - only where it makes contact). With both of these suggestions I omit the piping get and use a minimum amount of color (penned or brushed - I want the paper to be as dry as possible when I put it on the cake).

Hope this helps.