Preheat oven to 350 degrees. Grease a 9 x 13" pan. Grease a large piece of foil (large enough to cover pan).
Combine mochiko, sugar, water, and coconut milk. Remove 4 cups of batter and tint with food coloring. Pour into prepared pan and bake for 15 minutes.
While first layer is baking, remove 4 cups of batter and set aside. To remaining batter, add food coloring if desired. Omitting food coloring will result in a white middle layer, which is nice. Pour over first layer and bake 30 minutes.
Add food coloring to batter set aside. Pour over 2nd layer. Cover pan tightly with greased foil and bake for 1 hour.
Remove foil immediately. Cool completely before slicing with plastic or teflon-coated knife.