Melt 2 cups (one 12-ounce bag) each red and blue candy melts in separate bowls. Pour each color onto a parchment-lined baking sheet and spread evenly with an offset spatula. Refrigerate until set, about 20 minutes.
Break the red and blue candy into small irregular pieces.
Arrange the candy on a vanilla-frosted cake, gently pressing the pieces into the frosting.
To melt the candy melts, put them in a microwave-safe bowl. Microwave at 50-percent power in 30-second intervals, stirring, until smooth. (For a thinner mixture, melt with a little coconut oil.)