I've been watching this video from Cruft 2015, that's a demo from obedience winners.
If you fast forward to ~10' there's the scent discrimination exercise.
It seems that the correct cloths have a red mark on them. Is it always the case? Won't the dog just pick that one all the time then?
Edit: again in this video where the correct cloth is clearly different from the others.
Dogs cannot see red... it would appear as a light to dark shade of gray, depending on the intensity of the red.
But a sense of smell is by far a dog's strongest sense, and a dog will rely on that by default more than their sense of sight. My dogs are in Canine Scent training, and it is amazing how they can pick up an odor that is being carried by air currents. Much of our training is for me to learn to pick up on the cues that a dogs gives me when they are in odor. I believe that a dog trained to search for scents would follow the odor long before they notice a marking.
