It is most likely that the discoloration you are noticing is completely normal. It is quite common for the labia and the area right around the opening of the vagina to be more darkly colored than the surrounding skin. Depending on your ethnicity, the color can range from light pink to dark brown or purple. The color can also change as you age. Therefore, unless you have other symptoms this is probably just your normal variant.
On the other hand if you do have symptoms, such as thick or malodorous vaginal discharge, itching, or pain, then it is possible that you have a vaginal infection
, such as a yeast infection or bacterial vaginosis, that is causing some vaginal irritation and changing the color of the surrounding skin as part of an inflammatory response. If you have the symptoms, the best thing to do would be to visit your primary care doctor
or your OB GYN doctor to determine if your symptoms need to be treated.
If you have itching and rawness of the skin surrounding the vagina, another possibility would be lichen simplex, which is a chronic inflammation of the skin caused usually by various personal care products. If you think this might be going on, then again the first step is to talk with your doctor.