The piece will look nicest when it's up against a light source, but you'll be able to see the colors if it's up against something white, too. You can put it on the side of a cake against white icing and it will be visible.
You can also use this technique with royal icing that's well-dried as the framework, but the gumpaste will be fastest because you don't have to wait for it to dry. Using a pattern as a template and forming the framework over that will give you a design to work from if you don't want to do it freehand.
Kara Buntin owns A Cake To Remember LLC in Richmond VA, and cake supplies online at www.acaketoremember.com and www.acaketoremember.etsy.com