What is the ruling concerning my prayer if I considered what I saw of a yellow color at the end of menstruation to be yellowish discharge (ṣufra), and thus did not perform ghusl, given the presence of doubt as to whether that was yellowish discharge (ṣufra) or vaginal discharge that sometimes includes a yellowish tint?

The difference between yellowish discharge (ṣufra) and vaginal moisture (ruṭūbāt al-farj) is explained in Fatwa no. 121284. If the matter is unclear and the woman doubts whether her menstruation has ended or not, the default is that the menstruation continues until she is certain of purity. Therefore, she should not perform ghusl until she is certain of purity.