You can start out by making your favorite cake recipe (whether it be a mix or scratch) for WHITE cake. Be sure to use egg whites as specified on the back of the box and NOT whole eggs. The yolk gives a yellow tint and it will be harder to get nice, true colors. If you are baking from scratch, be sure your recipe makes enough batter for the number of cupcakes you need. My measurements are based off of making 24 cupcakes.
After you have your batter mixed up, split up the batter evenly into 6 different dishes. I got somewhere close to 3/4 cup worth of batter into each color. But mixes do vary some.
Then take your food coloring (I prefer Americolor) and add a few drops and mix the color in. You can see how dark my colors are once mixed. If you do them lighter, they will bake up lighter. But these baked up nice and bright as is…
And continue on with the remaining colors (remember to continue to work backwards). Here they are finished & ready for the oven.