1 tablespoon sprinkles and/or edible glitter, to garnish
In a bowl, mix together mozzarella, gruyére, and swiss cheese. Divide between four 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 two pieces butter-side down on the griddle. Top with cheese and cover with the other slice of bread, butter-side up. Grill for two to three minutes until cheese has started to melt and bread is toasted. Flip sandwich and grill for another two minutes.
Remove sandwich from grill and top with another smear of butter. Cover with sprinkles