In a bowl, mix together mozzarella, gruyére, and swiss cheese. Divide between 4 bowls and tint each with a different food coloring.
Slice brioche and heat up a griddle or skillet to medium-high heat. Meanwhile, spread butter on one side of the four pieces of bread.
Place 2 pieces butter-side down on the griddle. Top with cheese and cover with the other slice of bread, butter-side up. Grill for 2 to 3 minutes until cheese has started to melt and bread is toasted. Flip sandwich and grill for another 2 minutes.
Remove sandwich from grill and top with another smear of butter. Cover with sprinkles.