Prepare cake mix according to directions on the box (do the egg whites version over whole eggs, if there are two options), then divide the batter evenly into 6 bowls.
Using food coloring or paste, mix these 6 colors: red, orange, yellow, green, blue, and purple.
Prepare cupcake pan(s) with 18 paper liners.
Put each batter into a zip-top sandwich bag and cut off one corner to pipe with. It’s best if you cut one bag at a time, so the cake mix doesn’t spill out as you layer the batter.
Squeeze the colored batters, one at a time, into the paper liners. Pipe the edges of each cupcake first, right up against the cupcake liner, then fill in the centers. Pipe colored batter in the order the rainbow: red, orange, yellow, green, blue, purple. (If you want red on top, like a real rainbow, start with purple and go backwards. Updated pictures are below the recipe card.) Smooth each layer of batter with your finger before moving on to the next color. *TIP: Do all the red batter first, then all the orange, etc.
Once all the layers are finished, bake the cupcakes at 400 degrees Fahrenheit for 11 to 13 minutes. Baking them at a higher temperature forces the cake to grow upwards and bake quicker, so it doesn’t have time to spill over the top and get sunken in the middle.
Prepare The BEST Cream Cheese Frosting. Beat in half of an 8oz tub of whipped topping with a handheld electrical mixer. Fit a piping bag with a coupler (no tip or band) and then fill the bag with the frosting.
Pipe the frosting on top of each cupcake from the outside in towards the middle. Sprinkle with rainbow sequins. (*Store cupcakes in an airtight container in the refrigerator up to 4 or 5 days.)
*Optional: To save time, spoon the colored batters into the cupcake liners instead of squeezing the batter from bags, and spread each layer of batter with your finger.