Why do dogs have black lips?

Not all dogs have black lips. I've seen pink lips, brown lips, even
dark blue lips. Lip color varies by breed.

Why do women wear lipstick? 

Does anyone sell a lipstick for dogs?

To prevent sunburn.   Most of a dog is covered in hair & protects from
sunburn.   The nose & lips are mostly hairless and are usually black
due to higher levels of melanin.   Some dogs have pink lips and a few
have pink noses because humans have bred them for particular traits
for centuries, skewing the evolutionary advantage of particular
characteristics away from what would be natural.