Given that the moch dachuk was inserted for a very long time (possibly causing irritation) and that the color faded overnight (or was perhaps originally inspected in sub-optimal lighting conditions), there is room for leniency and your hefsek taharah remains valid.
Please note that the moch dachuk should not be left in for such a long time, precisely because its remaining in place can lead to irritation or even bleeding. In the future, perhaps you could slip away to a bathroom to remove it to a purse, even if you would only be able to evaluate it later on.
Not all stains make a woman niddah. It may prove helpful to you to review the laws of stains, as by reading our site's article, "Ketamim."
Please write back with any further questions.