I'm glad you all like it.
Really, it is very simple to make. You can take a white cake mix (we made ours from scratch).. and mix it up according to the directions.
When done mixing - separate the batter into 6 bowls.
We used GEL food coloring and dyed them according to the rainbow - red orange yellow green blue purple.
The key is in how it goes in the pan. Do not stir, just glob/pour. We did each pan opposite. i.e., if I started with red in the 1st pan, red was last in the second pan.
So take red batter and just pour/spoon as much as you think. Then, directly on top pour some of the next color, then, directly on top of that, pour some more of the next color etc... So it just looks like a puddle of colors in concentric circles (like a bullseye). DO NOT mix, stir etc.. Try to separate the batter evenly between the two pans so your cake comes out evenly. (I don't measure, just do)
Bake as normal or as directed on the package - and you'll end up with a rainbow cake too!