What Causes The Change In Color Of The Vaginal Lips?

Hi! can you please answer me! I am 23, in marriage 2 years. Before few days i notice that my vangina lips changed color but there is not any other symptom : itching or redness,
discharge( white thing) nothing just it's not pink anymore now is like bright brown i don't know when that happen because i did not checking like a month before now, can you please advise me what to do or should i visit a doctor I would like to miss it if it's no need to go thank you very much K