For Valentines' Day, I took a spin off of omnomicon's ranbow diet cake shown here:
For Valentine's day, I made a regular box yellow cake mix and followed the directions. Then, I divided the batter into 3 bowls. I used gel food coloring (better for you than the old stuff - and brighter!) and dripped it into the bowls to make pink, red, and left the yellow cake as is.
I greased the pans w/ crisco and flour to make it easy to flip them out to decorate.
One cake mix = 2 heart pans. I poured the majority of the red on the bottom of the first one, half of the yellow cake mix on top, and about 1/3 of the pink on the top. Then with the other, I did the opposite. Don't mix the colors, just dump on top of each other.
I baked for the normal time on the box directions, they came out just right.
I flipped the first one onto the cake platter and iced it.
Then I set the second layer & frosted it.