Don't know what help you're expecting. The juice stains your mouth. Brushing and mouthwash for your teeth. Soap and water for your lips.
Or stop using that juice.
I remember early on in vaping when most tanks were still plastic I had a chocolate flavor attack and dissolve the plastic tank as well as the black plastic drip tip. I'm sure your tank is fine but the drip tip is still plastic. I would remove the drip tip and let it sit in a little chocolate ejuice then wipe with a paper towel to see if you are removing dissolved plastic. Also try using a metal drip tip and see if that stains your lips... Just a thought.