Food coloring -- red, orange, yellow, green, blue, and purple
Preheat oven to 350 degrees F.
Combine all cake ingredients and mix with a mixer until combined. Do not overmix, but make sure all dry ingredients and wet ingredients are well-combined.
Divide cake batter evenly between six bowls.
For each bowl, add 1-2 drops of food coloring and mix. If more color is desired, add additional drops of food coloring until desired color is reached. Repeat with each bowl.
Generously spray a standard sized Bundt cake pan with nonstick cooking spray.
Starting with the red batter, spoon cake batter into bottom of pan. The cake batter should go around the bottom of the pan in a complete circle.
Carefully spoon the orange cake batter into pan, placing it directly on the red cake batter, once again making sure it goes completely around the red cake batter. Try not to mix the different colored cake batters; simply spoon one on top of the other. Repeat with remaining colors in the following order: yellow, green, blue, purple.
Place in oven and bake for 50-55 minutes, or until toothpick inserted into cake comes out clean.
Remove from oven and allow to cool in pan, on a wire rack, for 10-15 minutes.
Tap the pan firmly a few times on the countertop, gently shaking it to help loosen the cake. Carefully invert the pan onto a cake plate and lift the pan off of the cake. Allow the cake to cool completely prior to adding glaze.
To make glaze, place powdered sugar and milk into a bowl and stir well to combine. Add vanilla and stir to combine. Glaze should be thin enough to spoon over cake, but not runny. If glaze is too thick, add additional milk, one teaspoon at a time, until a spoonable consistency is reached.