2010 Cook's Illustrated Holiday Baking issue.
Before starting to mix the batter I learned that by using buttermilk and vinegar along with a bit of cocoa, that a chemical reaction occurs between these ingredients to give the natural red colour. Additional red food colour is used to intensify the colour.
First photo shows the basic batter. Second photo shows mixing in the red colouring. You mix the red food colour and the bit of cocoa powder together into a paste and then add it to the batter. Somehow that keeps the cocoa in suspension while baking resulting in a more even colour.
For a recipe for Red Velvet Swirl Brownies visit Fizzy Thoughts.