Cut ten 1-inch circles from colored paper. Cut an 18-inch length of string. Coat one side of five circles with glue. Evenly space circles, glue sides up, on a work surface. Lay string across centers of circles; adhere remaining circles on top, sandwiching string in between. Tie ends of string to straws or wooden skewers; insert straws or skewers into cake.
If you like, wear plastic gloves to keep from discoloring your hands.
Measure the height of the frosted cake. The size of yellow, blue, and red circles when stacked should be the same height. Adjust cutter sizes accordingly.