A light brown (like the color of coffee with milk or lighter) with no reddish tint is not a niddah color. It would not invalidate a moch dachuk, or any bedikah.
Additionally, we follow the halachic position that anything that can be easily removed and doesn’t leave a residue does not invalidate a moch or bedikah, regardless of color. Therefore, the first hefsek and moch you did are valid. The second hefsek, which you did the next day, counts as the bedikah of the first day of the seven clean days.
It’s possible there is some leftover blood or discharge in your vaginal canal that the moch dachuk caught. In the future, we recommend cleaning yourself out gently, either by soaking in a warm bath or using water or a wet cloth cloth at least fifteen minutes before doing the hefsek to remove any leftover blood.
Please write back with any further questions.