No, you don't need to because the batter will naturally turn pink due to the combination of the vinegar and the buttermilk. However, if you want the color of the donuts to really pop, consider adding a few drops of food coloring. Keep in mind though that if you are using regular food coloring that you may not get the red you want. I have found that gel food coloring is what really helps your colors pop instead of the traditional kind.