1Heat oven to 350°F (325°F for dark or nonstick pan). Spray bottom only of 13x9-inch pan with baking spray with flour. Make and bake cake mix as directed on box for 13x9-inch pan, using water, oil and egg whites. Cool in pan on cooling rack 20 minutes.
2Meanwhile, in 3 separate bowls, pour 1/3 cup of the boiling water over each flavored gelatin; stir until gelatin is dissolved. Poke warm cake every inch with wooden skewer halfway into cake, twisting skewer back and forth. Pour each color gelatin randomly over cake, allowing gelatin to fill in holes. Cool completely, about 1 hour.
3Frost cake. With decorating gels, draw vertical lines 1/4 inch apart on frosting, alternating colors. Pull fine-tip paintbrush in straight line across all colors. Repeat, working back and forth from one side of cake to the other to create a tie-dye effect.
4Roll fondant to 1/8-inch thickness; cut into rounds and flower shapes. Insert toothpicks halfway into cutouts; decorate cake slices as desired.