Use a primary color set of fondant to create a rainbow cake topper
1. Create rainbow colors (red, orange, yellow, green, blue, and purple) by mixing primary colors in a 3:1 ratio. Keep fondant covered while not using, so it doesn't dry out.
2. On a clean surface, roll 1/4 of each color into long, even ropes. Line the colors up in rainbow order.
3. Dab a slightly damp finger or paint brush on the ropes to glue them together (don't use too much water, or the fondant will adhere to the work surface).
4. Bend the ropes into a rainbow shape.
5. Use a knife to trim the edges in an even layer. (Cover scraps immediately so they can be re-used.)
6. Transfer to a piece of parchment paper or wire rack until firm - at least an hour, but ideally overnight.