Divide the shredded cheese into three equal bowls (about 8 Tablespoons per bowl).
Add a few drops of red dye to one bowl, green to the second, and leave the third plain. Stir the dye to dye the cheese.
Spread one slice of bread with mayonnaise.
Place bread mayonnaise-side-down into hot skillet; add 2 Tablespoons of each "color" of cheese to the bread and then spread other slice of bread with mayonnaise and place mayonnaise-side-up on top of cheese.
Cook until lightly browned on one side. Flip over and cook until cheese is melted.