Spray metal muffin tins with non-stick baking spray. Start by piping a half inch purple circle in the center of 15 muffin cups. Next, pipe a blue circle of batter around the center circle. Then add a green circle, then a yellow circle, an orange circle and finish with a red circle. You should completely fill the bottom of the muffin cup with colored batter. Repeat this process in each muffin cup, adding a few layers of batter until each cup is filled a little over half full.