All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The method of purifying clothes from impurity is to remove the impurity itself from the place and then wash the place by purifying water until no trace of its smell, or taste, or color remains. This is because if any trace remains, it would be evidence that the impurity is still in the place, as stated by the scholars.
However, if the color is difficult to remove from the clothes that were touched with impurity, then purity is achieved even if the color remais; this is because of the hardship of removing it. The Kuwaiti Fiqh Encyclopedia reads:
"The majority of the scholars among the Maaliki, Shaafi’i and Hanbali Schools are of the view that removing the color of impurity, if it is easy, it is an obligation to remove it because its existence proves that the impurity itself is still there. However, if it is difficult to remove the color or it is feared that the cloth will be damaged, then the place is purified by washing it and the existence of the color does not harm.” [End of quote]
Allah knows best.