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Preheat the oven to 350°
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Grease six 8" round cake pans, line with parchment paper
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Prepare a double batch of cake mix according to package directions, using water, vegetable oil and eggs
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Divide the batter evenly between 6 bowls (1 1/3 cups of batter in each bowl)
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Stir 1/8 tsp each of blue, green, yellow and purple food coloring into 4 of the batters (you can make purple by combining red and blue)
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Stir 1/2-1 tsp of orange food coloring into the 5th bowl (you can combine red and yellow to make orange)
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Stir 3/4-1 tsp of red food coloring in the 6th bowl of batter
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Pour the batters into the prepared pans
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Bake the cake layers at 350° for 13-15 minutes, or until a wooden toothpick inserted in center comes out clean
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Cool in pans on wire racks for 10 minutes
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Remove cakes from pans to wire racks and cool completely (about 1 hour)