For tie dyed cakes:
First, mix up your favorite white cake batter. The colors turn out a little better if it's a true white cake, but I tried it with a white cake recipe that uses egg yolks and it still turns out awesome.
Divide your batter evenly into six bowls (or however many colors you'd like - for camo, divide into two bowls, either vanilla and chocolate or dye the batter the two colors you'd like to use). Dye each batter the desired color.
Then add to the pan. For rectangle/square pans:
You really just get to be creative :) Take about 1/4 cup of batter and start plopping it around. Once all your batter is added, gently jiggle the pan to even it out.
Take about 1/4, start with one color and pour directly in the middle of the pan. Add the second color directly on top of that, alternating until all batter is added. Gently jiggle to even out.
Bake according to your recipe's directions, and you end up with something like this :D
(btw the colors are much brighter than this picture shows. Really pretty!)