Mix your white cake mixes together with the eggs, water and oil it calls for (to get your cake as white as possible, do the egg white only version). Set aside two cups of the mixture and refrigerate the rest of the mix until needed.
Tint the two cups of batter that you set aside with your food coloring to your desired color. Place your cake pop molds on a baking sheet and oil your cake pop cavities (top and bottom). Fill your bottom cake pop pan with batter, attach the top, and press all around thoroughly to make sure the parts are sealed together. Bake your tray in the oven at the temperature the cake mix calls for until the cake pops are almost cooked through. Just keep checking with a toothpick after 10 minutes until the toothpick comes out almost clean (mine took about 17 minutes at 350°).
Remove the cake balls from the pan and get the rest of your cake mix out of the refrigerator.
Place 6-7 cake balls into the bottom of a greased 6" cake pan and spoon in batter just until the cake balls are covered. Bake until a toothpick comes out clean, but try more than one spot when you test in case you happen to hit a cake ball that's already done instead of the new white batter underneath. Mine took about 35 minutes in a 350° oven. Repeat until you have three layers of cake. You can either do this one at a time if you only have one pan, or you can make multiples at once if you have several 6" pans. Allow the cakes to cool completely on a cooling rack before icing.
If you have rounded cake tops, use a bread knife to even out the tops of your layers so they will sit evenly on each other. Then stack them together with a layer of icing between each. I made the mistake of trying this before the tiers were totally cool with a lighter whipped icing option, and the layers were sliding around all over the place! The second time I let them completely cool and used the thicker regular icing and it worked much better. Once your layers are stacked, ice around the tops and sides, and you're ready to cut the cake and see the surprise inside!