Praise be to Allah.
If what is meant by the yellow discharge is the maniy (sexual discharge) because it tends to be a yellowish colour in women, then there is a difference of opinion among the scholars as to whether it is taahir (pure) or not. The more correct view is that it is taahir, because of the hadeeth narrated by Muslim which states that a man stayed the night as a guest at the house of ‘Aa’ishah, and in the morning he washed his garment. ‘Aa’ishah said: “It is sufficient, if you can see it, to wash the place where it is, and if you cannot see it, just sprinkle water over it. There was a time when I used to rub the garment of the Messenger of Allaah (peace and blessings of Allaah be upon him), then he would pray in it.” (narrated by Muslim, no. 288)
Al-Nawawi said in Sharh Muslim (3/198):
“Many scholars said that maniy is taahir. This was narrated from ‘Ali ibn Abi Taalib, Sa’d ibn Abi Waqqaas, Ibn ‘Umar, ‘Aa’ishah, Dawood and Ahmad in the more correct of his two views; it is the madhhab of al-Shaafa'i and the scholars of hadeeth…
See Fath al-Baari, 2/332
In Fataawaa al-Lajnah al-Daa’imah li’l-Buhooth (5/381) it says: The emission of maniy due to an erotic dream and so on does not make the clothes worn by the person naajis even if it gets on them, because maniy is taahir. But what is prescribed is to remove whatever gets onto the clothes for the sake of cleanliness and removing dirt.
But if what is meant is the yellow discharge that comes during menstruation and sometimes afterwards, as narrated in the hadeeth of Umm ‘Atiyah – “We used not to count the yellowish and brownish discharge as anything” (narrated by al-Nasaa’i, 368) – this is one of the things concerning which there is no dispute that it is naajis. Shaykh Ibn ‘Uthaymeen said in al-Fataawa (1/291):
What is known among the scholars is that everything that comes out of a woman is naajis apart from one thing, which is the maniy. The maniy is taahir; apart from that every other substance that comes out of the two passages is naajis and breaks wudoo’. On this basis, everything that comes out of a woman is naajis and necessitates wudoo’. This is the conclusion reached following discussions with some of the scholars and consulting references. But I am still not entirely sure, because some women have this moisture all the time, and if it is continuous, then the solution is to do the same as one who suffers from urinary incontinence: she should do wudoo’ for each prayer when the time for it begins, and do that prayer. I also spoke with some doctors and it became clear to me that if this discharge comes from the bladder, it is as we have said; and if it is from the vagina, it is as we have said, she should do wudoo’, but it is taahir and the things it gets onto do not have to be washed. And Allaah knows best.