Is it necessary to repeat the prayers from the sixth night, when purity was observed, and before the yellowish discharge appeared on the seventh day, or is what was done—repeating the prayers from the afternoon of the seventh day—correct?

The blood and yellowish discharge you saw immediately after performing ghusl (definition) (ritual bath) are considered part of your menstrual period. Therefore, you are not required to perform ghusl or pray until the yellowish discharge ceases. Your ghusl before the return of the blood was valid, but what you saw afterward is menstruation, and your tayammum (definition) (dry ablution) and prayers in that state are invalid. What is obligatory upon you is to perform ghusl after the yellowish discharge stops and you observe purity. Tayammum is only prescribed when water is unavailable or when one is unable to use it.