In advance: Make fondant letters. Tint 1 in. balls of fondant to match baking cup colors. Knead 1/8 teaspoon of Gum-Tex into each color (use orange for peach cups). Use mold to make letters. Unmold and let dry on cornstarch-dusted board at least 24 hours.
Bake and cool cupcakes in mini cups. Pipe tip 12 swirl; insert matching color letter.