My Red Velvet post got an interesting response from my friend Valerie, who bakes a family-heirloom recipe red velvet cake. Her son wanted a blue velvet cake one year, so Valerie used blue food coloring in the same proportions as she would have for a red velvet, but she said that it came out a really nasty shade of blue/green. Baby poop green was the actual description (and don't pretend like you don't know what that color is.)
We were talking about why it might have happened, and we figured that it must have been some kind of a reaction between the acid in the buttermilk and the food coloring. Also, her son wanted a bright sky blue kind of blue, and with the cocoa in the recipe you won't get that bright a color since the batter is light brown to begin with. Valerie's recipe is different than mine, too, and she said that particular cake didn't rise well, so that's why we suspected the acid had something to do with it.
I decided to try it today to see what would happen with different colors, and see if I would end up with such a beautiful color as she described. I used basic food coloring, which is what is basically available to home bakers, even if you buy it in larger bottles. I put about 1 1/2 tsp of color in each one, which would end up being the same proportions as the original recipe calls for.
One pan was blue, one was green, one was orange and one was yellow. The only color that looked nastier than I thought it should, outside of the fact that I was grossed out by the amount of food coloring I was putting into these in the first place, was the yellow one. It was more mustard than yellow, since the batter added a lot of brown to the yellow.
When they baked up they all looked darker than they should have from the outside, but when I cut them they were pretty bright. The colors were on the dark side, because there's no way that you'll be able to get a bright color based on the original batter, but they were definitely the "right" color for what I had intended them to be.
They tasted the same as regular red velvet, so at least my kids will have some cake to eat today, which is actually an unusual thing around here.
My suspicion about the baby poop green cake is that Valerie's recipe has different proportions of the cocoa or food coloring than mine does. That would affect the final color, and I have to add that when I was mixing the blue batter it did have a moment when the color wasn't so blue, shall we say. Valerie also said that this was quite a while ago that she made that cake, so food coloring formulations might have changed since then. Some colors are more stable than others, but it's usually the reds that change the most.
If I had used less color it definitely would have been more of the blue/green/brown, and a lot less blue. So if you decide to do a different color than red, make sure that the batter looks the shade that you want it to be before you bake it. Or just do it without the food coloring completely and do yourself a favor.