choosing a red food color for red velvet cake
Originally, red chocolate cake did not have artificial coloring. The red (not nearly as red as we're used to now) came from natural (not Dutch processed) cocoa (acidic), plus another acidic ingredient (buttermilk or vinegar). The double-dose of acid  reacts with the anthocyanins (antioxidants that are red, even more so in the presence of acid) in the cocoa and the alkali baking soda to create a subtle reddish tone. 

Try an old-style recipe, with natural cocoa and additional acid. Then if you must make your cake redder, it shouldn't take a bottle and a half of #40 red. Dutch processed cocoa will not give you the red effect, and it will cause your cake to be much darker. The flavor will be fine, but it won't be red and it will be difficult to dye red. Using dye to color a very dark brown (think Devil's Food) cake is going to require (as you have apparently seen), A LOT of dye. There comes a point that you're just going to cause anyone who eats the cake with their hands to have red fingers.

As far as your cake with a bottle and a half of food coloring? I'd quit trying to achieve a redder tone. You may or may not be able to taste the color in the cake, but you're going to see bright red tongues and possibly other weirdness.

This is an article about the chemistry of red chocolate and red velvet cakes: Chenected

Oh, and there is no such thing as too little artificial coloring for the sake of flavor, you can even skip it entirely. Your cake won't be as red, but it will taste just fine.


  The unrelated addition of “red” to a chocolate cake’s name initially arose due to the chemical reaction of acid in unsweetened bar chocolate and natural (non-alkalized) cocoa powder in conjunction with an acidic liquid (generally buttermilk or sour milk) with an alkali (baking soda), which reveals the red anthocyanin, a water-soluble vacuolar pigment. (In contrast, chocolate cakes made with baking powder and a non-acidic liquid turn out blackish in color.) Near the beginning of the 20th century, these chocolate cakes became known as “red cake,” “red regal cake,” “red feather [as in light-as-a-feather] cake,” “feather devil’s food cake,” and “red devil’s food cake.” However, their slightly reddish-dark brown hue was quite different from and much duller than food coloring-enhanced red velvet. - Tory Avey History of American cakes