Tissue from the uterine lining does make a woman niddah, just like blood. However, it is possible that the color of the tissue you are describing is not a niddah color, in which case it would not push off the clean days.

We recommend soaking in a bath as part of the cleaning prior to making a hefsek taharah, so that there is less chance of having old blood interfere with the clean days. We also recommend, in your case, that you make early attempts at a hefsek taharah and bring the cloths for evaluation, in case the color is indeed acceptable. Even some browns can be accepted on a hefsek taharah! If you'd like to share your location with us, we can try to make a referral to a local halachic authority.

Please write back with any further questions.