1Heat oven to 375°F (350°F for dark or nonstick cookie sheets). In large bowl, beat cake mix, shortening, butter, vanilla and egg with electric mixer on low speed 30 seconds, then on high speed 1 minute.
2Divide dough into 4 equal parts. Roll each part 1/8 inch thick on lightly floured cloth-covered surface with cloth-covered rolling pin. Cut with 2 1/2-inch cookie cutters into desired shapes; sprinkle with sugar. On ungreased cookie sheet, place 2 inches apart.
3Bake 5 to 7 minutes or until light brown. Cool 1 minute; remove from cookie sheet to cooling rack. In microwavable bowl, microwave frosting uncovered on High 20 to 30 seconds or until melted; stir. Frost cookies. Stir together small amounts of water and food color. Paint colors on freshly frosted cookies, using fine-tip brush, then swirl colors with brush or toothpick to create marbled designs. Dry completely before storing.