This subject has always fascinated me.

Pink nose is technically called "hypopigmentation" (or snow nose) and is a result from loss of sunlight. It does not have anything to do with snow on the ground or the days being colder. You will notice that it begins a little after all the leaves on desiduous trees have fallen. The same way that lack of sunshine causes trees to signal it's time to change color, it signals to dogs to loose some nose pigment. Once winter is over pigment darkens again but it doesn't always return as dark as it was initially. In older dogs the condition seems to be a little worse every year. This is because the nose never returns fully to the full dark pigment it was when they were younger.

Another reason for this condition is a deficiency of B vitamins, PABA in particular. A lot of people who show their dogs in breed (conformation) supplement during winter to help offset the Snow nose condition. It has been known to help.