Mid-cycle staining such as you describe is often the result of the dip in the estrogen level at the time of ovulation, which weakens the uterine lining. It need not make a woman niddah, so long as she is knowledgeable about the laws of stains.
We recommend that you keep a record of your menstrual cycle, including days of spotting, so that this can be taken into account when preparing for your wedding.
Once you are married, if you have staining during the seven clean days, you should discuss the situation with a halachic authority. Questionable colors can be evaluated, and if necessary the scheduling and number of bedikot can be adjusted to avoid difficulties..
Given your concern, you may wish to discuss the staining with your physician in order to rule out other causes.
Please write back with any further questions.