Purity: Characteristics of Istiḥāḍah
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 390:
One of the types of blood that women discharge is the blood of istiḥāḍah. A woman at the time of experiencing istiḥāḍah is called a ‘mustaḥāḍah’.
Most of the time, the blood of istiḥāḍah is yellow in colour, cold, comes out without pressure or a burning sensation, and is not thick. However, it is possible, sometimes, for it to be black or red, warm, thick, and to come out with pressure and a burning sensation.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)