I made these yesterday for the family, and they were a big hit!
For these-I just used a box mix (white cake) and cream cheese frosting. I divided the cake batter into several small bowls (depending on the colors/how many you want to use) and used food coloring to dye the batter in each bowl a different color.
I then layered each color, one after the other into the cups…and baked according to the box mixed I used. Very easy to do, just takes a little time & patience.