I wanna know I work in Mcdonalds were I'm constantly moving and I wear slip resistant shoes were there is no ventilation in the sneaker. It doesn't bother me or has any pain.
Not the clearest picture but it may be a fungus. If it remains persistently discolor end then go see a podiatrist and get it analyzed to determine if it is a fungus, bacteria or combination thereof. You can always try applying some Vicks Vapor Rub nightly and see if that helps. Keep clean, dry socks as well to reduce moisture.