If your dog has been normally in doors and is now been spending a lot of time out doors that could be why.
I have a small mutt (shih tzu,
some kind of terrier dachshund mix,
Anyways he had been an indoor dog since he was a pup,
well my hubby and I bought our first home recently and finally had a decent backyard he could spend time playing in.
After about a week the skin around his male member began turning dark.
When the weather changed and began getting cold he began to spend more time inside and the skin in that same area turned back to a light pink...
We recently got a new puppy and the same thing appears to be happening to him.
In my opinion,
the reason I believe this happens is the skin is "protecting" itself from the elements.
Like if a dog is indoors a majority of the time it might not shed no where near as much if it where an outdoor dog,
since the temperature inside a home is neither too hot nor too cold.
The skin just knows it needs to adjust in different environments.