Make vanilla cake according to box instructions, then evenly divide your cake batter into 2 other small bowls, keeping the vanilla in the original bowl you mixed it in. Use red gel paste in one bowl and blue gel paste in the other bowl and mix until color desired.
Pour the white cake batter into a 9 x 13 cake pan, sprayed with non-stick spray. Then add the blue or the red by pouring on top, then swirl with a knife a few swirls, but not a lot, or you'll get other colors and it won't be red, white and blue.
Bake cake until done and let cool.
Once cake has cooled, make the frosting.