I would like to answer the practical point first that you do not need any kind of treatment for your pigmented genitalia, Man is hardly serious about the color of the vagina.
Yes feeling and tone is important for him and no doubt how fair vagina you have,if you are not careful and emotionally not well adjusted to him everything is useless.
Some increase in size of breast and it pigmentation is known after regular sex, may be some change in pigmentation at vagina also.
But as a rule that can't be drastic to be noted.
May be you are worried because you will face him soon, but color of vagina is the least important thing in a man's mind I can say.
And lastly masturbation
can also not be a cause if you are not applying any cream/lubes etc.