I do not recommend coloring a brownie mix to make red velvet brownies. The boxed mix is so deeply dark and rich, it would take A LOT of red food coloring to make them red & honestly I am not sure it would ever be red enough. Using a cake mix is the best option for making red-colored brownies. Brownie mix and cake mix are often the same exact ingredients. It is all about what you add to the boxed ingredients that make it a cake or a brownie. For example, fewer eggs make the texture fudgier like brownies.