Praise be to Allah.
What usually happens with women is that the menstrual blood comes first, then it is followed by brownish discharge, then yellowish discharge, then comes the whitish discharge which is called al-qassah al-baydaa’, which is the sign of tahaarah (purity). Some women find that the yellowish discharge ceases and they do not experience the white discharge after that; the vagina becomes dry. If the dryness persists for a complete day, more or less, then this is tahaarah, even if there is no white discharge. But if a woman’s usual cycle is to have blood then brownish discharge then yellowish discharge within a certain time limit such as seven days, then she finds that she is dry, then she should do ghusl and start praying – she does not have to wait a whole day. If a woman sees that she is taahir (pure), whether by noting the whitish discharge or the dryness for more than one day, then she experiences a brownish or yellowish discharge, that doesn’t matter, and she should not pay any attention to it; she should remain taahir, unless blood is discharged – because Umm Salamah (may Allaah be pleased with her) said: “We used to pay no attention to a brownish or yellowish discharge after becoming taahir.” Based on the above, you have to figure out what your cycle is, whether it is irregular or regular. The woman with an irregular cycle has to wait until she becomes taahir, unless her bleeding etc. lasts for more than fifteen days; if it reaches this extent and she did not become taahir, then she should do ghusl and start praying, and there is no sin on her in sha Allah. The woman whose cycle is regular should follow the details described above.