I am 28 year old women who has only just started to discover these changes. After reading up on it though I discovered it was caused by stimulation, it makes the blood run to you girl bits ( same as with a guy ) and over time this can cause permanent or occasional darkening of the area.
If you also have burning, itching, or the skin tears easily it can also be a sign of thrush or other irritation such soaps or deodorants, but if there are no other symptoms than it is most probably a normal and natural change.
I like most females on here was alarmed at first, and worried that it would cause my husband to lose interest however he says he hadn't even noticed until I mentioned it and now I have bought to his attention he says he doesn't care and it hasn't changed anything between us.
I think the guys on here are right and we care about it way more than our partners do.