Honey mixed with a little lemon juice helps to lighten dark lips.
Try changing your toothpaste brand as it may be allergic to you.
Mix 1/2 tsp Pomegranate juice with 1/2 tsp beetroot juice and apply regularly to cure dark lips.
Soak 2-3 almonds overnight. In the morning, peel its skin and grind it to smooth paste. Add a little fresh cream and 2-3 drops of lemon juice to it and apply it regularly to cure dark lips.