Light to dark--that's the idea! I call this cake simple because rather than having to split the batter before you color it (like a rainbow cake), you just color it a little at a time in one bowl!
Once your batter has been mixed and is smooth, begin by adding one or two drops of coloring to your bowl. Turn the mixer on, then stop it to scrape the sides and bottom of the bowl, and mix again until it is evenly incorporated. Now your first and lightest layer color is ready. You will want to come up with a measurement for the amount of batter used in each layer so that they turn out (somewhat) even. Dip the decided amount of batter into a pan and set aside. Now, go back to your bowl of batter and add one or two more drops of color. Turn the mixer back on until it is evenly incorporated again (scraping the sides and bottom of the bowl ensures that the batter is evenly colored). Dip up your next layer and move on to the next one! Repeat this process until you have used all of your batter dipping up the desired number of layers.
You want to be sure that you add enough coloring to each layer that a difference can be noticed, but not so much that you can no longer make the layers any darker. This happened to me on the last layer--soft pink can only go so dark! I ended up adding a tad of red and all was well. Just be aware of that tiny detail when you start!
Once all of your layers are baked and cooled, begin with your darkest layer on the bottom. Continue icing the cake as usual, stacking the next darkest layer. Once your cake has been iced between each layer, admire your beautiful work before you cover it up! It should be stacked lightest to darkest from top to bottom. Ice the outside of the cake and call it a day. You are sure to please the crowd with this sweet and simple creation.