Add ΒΌ cup of shredded mozzarella to 4 bowls. Add a squirt of food coloring to each bowl then mix with a fork until cheese is coated. Create rows of each colored cheese on a slice of bread then add the other slice on top.
Butter the outside of the bread and grill over medium heat until cheese has melted and the bread is golden brown. Let the sandwich rest then cut the sandwich perpendicular to the rainbow so that you can see the rainbow.