It's called a snow nose, see 3 below...
From Dr. Mike Richards:
The various causes of loss of pigmentation of the nasal planum (the usually black portion of the nose) that I can find good references for are:
1) Dudley nose -- loss of pigment seen in a number of breeds for unknown reasons in which the nose is black when dogs are young but fades to brown or sometimes even white as the dog ages.
2) A transient form of the above condition may exist or it may be something else, but some dogs do lose some pigment and then recover the normal coloring of their nose over time.
3) Labradors, Golden retrievers, Bernese Mountain dogs and Siberian huskies may have seasonal variation in the color of the nasal planum, usually lighter in the winter and darker in the summer. The cause of this is not known, either. Some vets refer to this as "snow nose".
4) Vitiligo can cause loss of pigment of the nose and it may also be part of a syndrome of immune mediated disease in which hypothyroidism also occurs, so perhaps this is the link to loss of color of the nose and hypothyroidism but if it is, the hypothyroidism is thought to occur after another disorder causes the color change. Dogs with vitiligo normally have development of patches of white hair or white hairs scattered in the hair coat. I don't think that loss of pigment in the nasal planum only is likely with this condition.
5) Contact dermatitis can cause loss of pigment in the nose -- some dogs are reported to be sensitive to the plastic that is found in some feeding bowls, for instance. Continual irritation of the nasal planum from a cause like this might lead to loss of pigment. Usually the lips are also are inflamed or may have pigment loss if they are dark, too.
6) We see dogs with discoid lupus and with phemphigus that have loss of pigment of the nasal planum, usually patchy but sometimes most of the planum is involved. There are also usually skin sores around the edges of the nasal planum, on the bridge of the nose, around the eyelids or places like that.
7) I have seen cats with squamous cell carcinoma lose the pigment in the nasal planum as an early sign. I am not sure that this same problem occurs in dogs but I wouldn't want to totally rule it out.
