How To Lighten The Lips To Get Them In Normal Color?

Hello Dr Batra. I'm hoping you can assist me. I can send you a pic of myself to help you better to help me if necessary. My skin is quite light, but since my teens, my lips have gotten progressively darker. People have even commented asking if I smoke, which I dont, but this is quite embrassingly. Is it sunburn or a vitamin deficiency? I've stopped using lipsticks as well hoping it would help. What could be causing it, and how can I lighten them to a more normal colour?