Use the method I learned in art school for oil painting brushes. If it works for oil paint, it should work for this.
First of all, never soak a brush top down in water, or you'll make it bend permanently and you won;t be able to paint with it. (For soaking brushes they have a spring that goes across the top of the jar and you hold them suspended in the liquid, by pressing the handle into the spring)
Anyway, no need to soak.
Rub the bristles on a bar of soap. Then rub the soapy bristles on the palm of your hand. rinse and do it again, until no more color comes out.
