Probably just a natural physical trait of the animal but the bigger question here is how do you know they are cold?!!!
Why Does My Dog Have Black Balls?
My maltese has black balls and he is white is that normal
I don't know mines a girl but my cousin has a blonde dog with black balls so yeah
Is your dog black? If he is then he will have black balls , but if your dog is blond then he may need to see a vet.