Put paint brushes you used to paint immediately after use in a pot with boiled linseed oil. You can put several in one pot. The brushes remain good for a year. For use only sweep cleanat the edge of the pot.
If there is water or white spirit in a brush is because these here has been exposed to water or turpentine you can do this there easily get out by gripping the brush with 2 flat hands and in the hair in the paint rapidly to rotate around just as one fire making with a wooden stick. The bristles take the paint and the water disappears into the paint without adverse effect. Thus there is no dye in the first portion of the applied paint on the surface.