Are the periods in which blood descended considered menstruation, was what the questioner did regarding prayer correct, and what must she do now?

What you saw first, the yellowish and brownish discharge, is not considered menstruation. Then, the blood for which you stopped praying is considered menstruation, along with any brownish discharge connected to it. What you saw after that depends on the difference of opinion regarding the minimum period of purity between two menstruations. The shortest duration for purity between two menstruations is thirteen days. If the period between the cessation of the first menstruation and the return of blood is less than this duration, then the returning blood is considered istihada (non-menstrual vaginal bleeding). However, the blood returning now after sixteen days is considered menstruation.