Why does my dog paw turn red?

My dog paw is suppose to be black but some times it turns red and i get worry!

It depends on what kind of red you mean. If it's likea bloody red i would suggest that the dog has either been chewing or licking it paw too much and this is commonly an act of boredom. But if it looks like it's inside that paw i would say hes walking on an unsanitary surface.