Is it permissible for a woman to act on her assumption and perform ghusl when unsure of the color of discharge after menstruation has ceased, and upon examining the area, if she believes what she saw was white moisture?

If the questioner is suffering from obsessive doubts, she should not pay attention to the delusions that occur to her regarding the attainment of purity. If she is not suffering from obsessive doubts, then she must not perform ghusl (definition) (ritual bath) from menstruation until she is certain and convinced of seeing the sign of purity. This is because the default state is the continuation of menstruation until certainty of its cessation is achieved. So, if she doubts whether purity has been attained or not, the default is that it has not been attained.