From babble.com

Spring Flower Cupcakes
makes 18 cupcakes
1 1/2 cups vanilla frosting (your favorite recipe)
food coloring or a natural food coloring alternative
vegan vanilla cupcakes swirled with green food coloring (natural alternatives work great for green!)
* you can also use your own fave vanilla cupcake recipe*
2 1/2 cups white pastry flour
1 1/4 cups vanilla soy milk
1 cup sugar
1/2 tsp salt
1 tbsp baking powder
1 tsp apple cider vinegar
1 tsp vanilla extract
1/3 cup canola oil
1/2 cup applesauce
Bake your cupcakes – 350 for 20 minutes.
Cool.
Split your frosting into three cups and swirl with different food coloring colors.
Add the frosting in layers to a decorating tube. One color right on top of the other.
When you squeeze out the frosting you will get different colors – but a few that swirl nicely together.