Because your lip's turning black.
4 seconds sound clip from the Next Friday (2000) movie soundboard.
You can hear this line at 00:29:26 in the Blu-ray version of the movie.
Quote context
[...]
- About to start a little puff, puff, give in this motherfucker.
- Why don't you come and sit next to your auntie?
- I know you smoke a little weed, right?
- Because your lip's turning black.
- Would you like some wine, baby?
- Well, how about a little cognac?
- Shit, I can roll a blunt better than them bitches in Havana.
[...]