The history of the original red velvet cake recipe goes all the way back to the 1800's. Back then cocoa powder was processed differently and the chemical reaction between it, the buttermilk, and the baking soda produced a natural reddish hue that is more intense than what we see today. Nowadays we use red food coloring to achieve the same result. This New York Times article has even more interesting history about it if you are curious to know more.