A vitamin B12 deficiency can cause your lips to turn dark. Having less than 200 pg/mL will cause your lips to turn dark. Your vitamin B12 level should be between 200 pg/mL and 900 pg/mL. You should consume vitamin B12-rich foods such as fish, soybeans, eggs, cheese, low-fat milk, red meat, silken tofu, trout, chicken breast, animal liver or kidneys, and many others.