> I thought it may have been the beet pulp in the food, but an article i read said beet pulp has no colour.
Depends on the actual pulp used. Beet pulp in it's natural form has no colour (bit like a turnip), but some beet pulp is molassed giving it a deep, dark reddish brown colour and often this is the kind used to bulk up dog food.
Is she licking her chest area? It may well be staining from licking and the bacteria on the area causes a red stain. You could try putting a no bite solution on the area, or using a bib if it will cover the area. One of mine gets food somehow on the chest area then proceed to lick it off and I put a snood on him for feeding now.
I have a dog who did start to lick an old wound site and it turned red-brown I mixed pure aloe vera gel with a couple of drops of lavender essential oil mixed in a put it on the area daily and he stopped licking it mainly because of the smell of the lavender which he hates.