Is the pink discharge that appeared one day before the expected period, and then stopped, considered menstruation that prohibits prayer?

If you ascertain the following day that what you saw was habitual blood, then it is menstruation. However, if you do not ascertain it, then the default is that menstruation is not established until certainty is attained. If you observe purity after these drops, then the 'Isha and Fajr prayers become obligatory upon you, and you must make them up after your purity.