Divide dough into 3 equal portions. Tint one portion red, one portion orange and one portion yellow. (Wearing plastic gloves protects your hands from becoming colorful!) Add 2 tablespoons flour to each portion during mixing.
Place one piece of parchment paper on work surface. Sprinkle with additional flour; add one portion of dough. Place another piece of parchment paper on top and roll dough until 1/4-inch thick. Remove top layer of parchment paper. Using 2 1/2- to 3-inch leaf-shaped cookie cutters, cut dough. Pick up remnant dough from between cookies, set aside. Repeat with remaining colored dough.
Collect all remnant dough and press into a ball to make marbled dough. Roll between parchment and cut as above. Sprinkle all cutouts with sugar.
Arrange cutouts 2 inches apart on prepared baking sheets. Freeze 10 minutes. Preheat oven to 350 degrees F. Bake 7 to 10 minutes or just until set. Transfer to cooling racks. When cool, decorate with chocolate-hazelnut spread to mimic leaf lines.