This red staining is often due to over-licking. Dogs will lick a lot if they are itchy, or painful. It's a good sign that he is walking normally, but it's always possible that this is the start of an infection, or allergy.
For now, it may be helpful to flush the area with cool water 2-3 times a day until it looks better. If you notice any swelling, or he starts to limp, then it would be best to have him evaluated by a veterinarian. It would also be good to have him seen by a vet if the foot does not look better in two weeks.