That's a great question! The red in red velvet cake is always derived from food coloring, which can be extracted from natural sources, such as beets (they produce a red compound called betanin) or chemically synthesized dyes. However, the reaction of acidic vinegar and buttermilk tends to bring out the color of red anthocyanin compounds that are naturally found in cocoa and many other flowering plants (like the reds in blueberries and apples). This may make the chocolate appear redder, especially if the cocoa is not "dutch processed," which means the cocoa has been treated with an alkali chemical to neutralize its natural acidity.