Bake and cool your favorite cake recipe or cake mix in prepared 9 x 13 baking pan. Arrange cake on foil-wrapped cake board. Spatula ice cake with white buttercream icing, making the top slightly thicker.
Using largest teeth on triangle comb, comb an arch in the top half of the cake top. Spray red, yellow, green and violet stripes with Color Mist. Spray the remainder of the cake top and the cake sides with blue Color Mist, allowing some of the white buttercream to show through.
Add tip 18 white buttercream zigzag puff clouds at the ends of the rainbow. Add tip 233 green grass buttercream bottom border.