Are you saying that her feet are red from licking them? Or do you think she has a food allergy to red and orange things? Beet pulp is not a red item at all and saliva stains are not related to what color is in the food. Those are myths. She may be licking her feet due to either contact allergy, like to grass or a cleaning product, or food hypersensitivity, which can be to any ingredient that has protein in it, including meats, grains, fruits, veggies. The stains can be a yeast infection due to being damp all the time or a bacteria in the saliva.
Definitely try switching foods, but take the ingredient list with you and try to find a food that is very different from what you are currently feeding. Also, start rinsing her feet when she comes in from outside. Some people keep a cat litter pan by the back door for this purpose. One cup of vinegar to one gallon of water is a good rinsing agent.
