Does a woman's vaginal discharge become impure when mixed with pre-ejaculatory fluid?

Summary of the Fatwa:

The discharges from a woman's uterus are pure. As for madhiy (pre-seminal fluid), it is impure and requires purification by performing wudu' (ablution) and washing the penis.

Liquids, such as madhiy and other discharges, if mixed with impurity, become impure, especially if the impurity is significant, unless the characteristics of the liquid change.

Ibn Taymiyyah says: The correct view is that water and liquids do not become impure unless they change [in color, smell, or taste].

It is preferable to pour out a very small amount of water if it mixes with impurity as a precautionary measure to avoid doubt.