ANDY!! You were RIGHT! Here's the Wikipedia thing I just found.
's 1972 reference, American Cookery
describes three kinds of red velvet cake varying in the amounts of shortening
used. All of them use red food coloring
for the color, but it is mentioned that the reaction of acidic vinegar
tends to turn the cocoa
a reddish brown color. Furthermore, before more alkaline "Dutch Processed" cocoa was widely available, the red color would have been more pronounced. This natural tinting may have been the source for the name "Red Velvet" as well as "Devil's Food" and a long list of similar names for chocolate cakes.