Take about a 1/4 cup of the batter and put into a small bowl and drop a single drop of food coloring of your choice into it and mix until it is evenly incorporated. (eatthelove.com)
Put a few drops of red food coloring in half the dough and blend well, so you have a white ball and a red ball of dough. (thefashionablehousewife.com)
Lastly, put a small drop of red food coloring in a bowl or plate. (lovingmomentsbras.com)