Prepare cake mix according to package instructions. Mine asked for the eggs, water and oil as mentioned in the ingredients list.
Add red food coloring to 1/4 cake batter.
Add red cake batter over the blue. Tip the pan and use a spoon to spread batter up the edge of the pan a bit.
Bake as directed (usually 40-50 minutes).
Frost and serve.
Note on food coloring: start with a few drops, stirring to see color depth. Add more a few drops at a time until you reach the desired level of color.