What is the ruling on seeing a red or pink thread when cleaning oneself (istinja'), while being unsure if it is blood or an ordinary thread?

Red and pink are colors of blood, and one must perform istinja' (ritual cleansing) from this thread. It is not considered menstruation unless, by its присоединение (joining) with blood before or after it, it reaches a day and a night, which is the minimum duration for menstruation according to the majority of scholars, and this is the relied-upon opinion for fatwa. So, if a woman sees this thread alone, she should purify the area, perform ablution, and pray.