How can one differentiate between menstrual blood and istiḥāḍah blood when the color of the blood is not the usual menstrual color, but it has the smell of menstrual blood, or when the smell of menstrual blood is present without the presence of blood?

The fundamental principle regarding menstrual blood is what a woman observes as blood, whether red or black, with or without an odor. A woman is not considered to have istiḥāḍah (non-menstrual bleeding) unless she sees blood at a time when it cannot be menstruation, and the maximum duration of menstruation is fifteen days. If a woman sees blood for five days (three black and two red), all of it is considered menstruation. It is not istiḥāḍah unless the duration exceeds fifteen days, or she sees blood before thirteen days have passed since the cessation of the previous bleeding. Yellowish and brownish discharges are considered menstruation if they are continuous with the blood. A woman is not considered to have istiḥāḍah unless she sees blood at a time when it is not suitable to be menstruation. In such a case, she reverts to her previous custom, or she acts according to sound discernment (black color, characteristic odor, accompanying pain). Discerning features are not considered unless istiḥāḍah is first established.