A good rule of thumb with any pet – especially any exotic pet (reptiles, birds, rodents, lagomorphs, etc) is if it is not normal for the pet, call the vet. The problem with exotics is they can go down very fast if you adopt a "wait and see" attitude. Even with more common pets, waiting to see what happens when there is something out of the ordinary could be the difference between less expensive and easier treatment or more expensive and harder or even life and death. Exotics can go down VERY fast. I had a rabbit with an umbilical hernia that began to enlarge. The vet said if that happened, get the rabbit it immediately – which we did. The rabbit still died. Our exotics vet at that time was up to 40 minutes away. If this is not normal for your pet, make a phone call and consult with the vet. There are things that cannot be handled on line – medical and serious behavioral issues are two. There are various things than can cause pigment changes in hamsters: infection (even fungal), injury to the region, hormones, allergies (food or some beddings), irritation, etc. Even a vet could not diagnose a medical problem on line without seeing the animal. Call the vet and discuss this with a medical professional that can see the animal, look for other symptoms that you may be missing and hopefully it is nothing serious.