I was wondering if there is a way to create a mask from an effects; let me explain.
I am currently using liquify on footage, and I want to isolate the part of the footage that has liquify on it (to put some color only on the liquify part for example).
A Difference Matte would probably get you the results you want. Original copy of the footage on the bottom layer, liquified version on upper layer, Difference Matte applied to the upper layer. Make sure your new color effects are after the Difference Matte effect in the render order, so that AE applies the effects in the order you want.