Vaginal discharges is a part of life for women. What you have been seeing for the last year or so is the mucus fluid that is produced by the lining of your vagina. It's purpose is similar to that produced in your nose. It traps foreign particles and moves them out of the vagina.
A light brown color indicates some blood is mixing with the mucus. It could be that you are about to have a menstruation and you are seeing the first traces of the lining beginning to be shed. Or it can be the sign of an infection.
Since you haven't had a menstrual bleeding yet, it is possible that it is beginning. An actual period will make you think you are bleeding internally. The fluid will be bright red or dark red and may have some clots in it. While the quantity is not huge, it is distinctly noticeable.
When menstruation first begins, it can be very erratic, both in frequency and amounts discharged. It will take one to two years before your body settles down into a steady rhythm.
What you need to do is start carrying pads with you so that you can use them when it does come.
If the brownish color continues for more than a week, you should see a gynecologist to make sure that you didn't pick up an infection.

