Is it considered postpartum bleeding (istihada) after the Asr prayer on the fifteenth day, and was it obligatory to perform ghusl at dawn on the fifteenth day and consider everything thereafter as istihada, based on white discharge tinged with light yellowishness, and the presence of yellow color on the underwear, while this contradicts another fatwa that considers yellowishness to be menstruation?

The criterion for knowing purity is to insert a cotton swab and for it to emerge clean of blood, yellowness (ṣufrah), or brownish discharge (kadarah). If it emerges as such, then perform ghusl and pray. If it emerges mixed with yellowness, then you are still menstruating. If you become pure and perform ghusl, then later see yellowness, do not consider it anything. However, if the yellowness continues for fifteen days from the beginning of seeing blood, then after the expiration of this period, you become a woman with irregular bleeding (mustaḥāḍah). There is another jurisprudential opinion that considers yellowness not to be menstruation at all, and this can be adopted when there is a need to alleviate hardship.