A woman is not obligated to perform a hefsek taharah on her fifth day. For example, if a woman knows she is still actively bleeding through sunset of her fifth day, she need not attempt a hefsek taharah on day five.
However, there is a halachic preference for a woman to become tehorah as soon as she is able. Also, regularly extending time spent in niddah a day longer than necessary can take a toll on a marriage.
When a woman has a similar color repeating each month, she can ask her rav if she can assess it for herself in the future, or if her cloths require evaluation each time. Sometimes, a simple step such as soaking in a bath on day five (or, doctor permitting, douching) can help produce clearer bedikah cloths sooner.
Please write back with any follow up questions.