I want to bake a white cake and the recipe I found on Joy of Baking asked to use cake flour and the egg whites are to be beaten until stiff. After folding in the egg whites, it says not to overmix but I was planning to make a rainbow cake. So if I add in color and stir until the colour is incorporated into my batter - will that be over mix?

I baked a rainbow cake from Martha Stewart's website but instead of baking 6 different colour cakes, I baked in 2-8in pans by mixing the colour in layer by layer but when it came out, the side was crusty and inside was pretty moist

I think you run the risk of deflating the batter if you mix in color after you fold in the egg whites.