Gel food colour - redorange, yellow, green, blue and purple
For the Frosting:
You can use your favourite frosting or see below for Swiss Meringue Buttercream
For the Cake:
Preheat oven to 350F and grease the sides and the bottoms of 6 8-inch pans (see my tips on what to do when you don't have 6 pans). Line the bottom with parchment paper for easy release. Set aside.
In a medium bowl, combine 3 cups flour, 1 tablespoon baking powder and ½ teaspoon salt. Set aside.
Using a handheld electric mixer or a stand mixer fitted with the paddle attachment, beat the 4 eggs, 2 cups sugar and 1 tablespoon vanilla until light and triple in volume (about 10 minutes). Start on low speed then gradually move up to medium-high speed. Drizzle the ¾ cup canola oil while beating.
Switch to low speed and add a third of your flour then half your milk. Another third of your flour, the rest of your milk and finally the rest of your flour. Make sure each addition is incorporated well. Beat the batter until smooth (about 5 minutes).
Weigh your batter and divide the total weight into 6. Transfer the corresponding weight into separate bowls (see my tips on how to correctly divide and transfer the batter). Tint each bowl in red, orange, yellow, green, blue and purple. Then transfer into prepared pans and bake for 20 minutes (see notes) or until a toothpick inserted in the middle comes out clean or with minimal crumbs.
Let cool in the pans for 10 minutes before transferring to a rack to cool completely.
Trim the tops of the cake to ensure they are level. Also trim the sides to remove any burnt spots. The important thing to remember is all your cakes must be of equal width and height.
Put the purple cake on a cake turntable first and evenly spread frosting on top. Do the same for the blue, green, yellow, orange and red layers. Then proceed to cover the rest of the cake with frosting and decorate as desired.
You can also use 6 6-inch pans. Just remember that you will end up with a smaller width cake but thicker layers, and therefore a taller cake.
If you have 6 pans, they won't all fit on the middle oven rack position. So at 10 minutes, switch the bottom pans with the top pans for an even bake.
You can cook the cakes the day before and make the frosting the day you're serving the cake. Just leave the cakes on the counter and loosely cover with a tea towel.
You'll notice that my red layer is more pink/coral than red. I used the red gel colour but it turned pink. Any tips?