Preheat oven to 350° or according to box instructions.
Prepare cake mix as instructed on box, substituting milk in place of water (same amount).
Divide the batter into three separate bowls.
Add a different color to each of the bowls (one red, one blue, one stays white).
Continue to add the red and blue in small amounts until each bowl reaches the desired hue.
Grease the cake pans and insert well. Beginning with one pan at a time, place the insert into the pan and pour alternating colors of batter.
Once filled, remove and rinse insert. Move to next cake pan and repeat the process.
Colors can be alternated as desired.
Bake according to package directions. Remove and let cool completely before frosting.