I just use hot water from the microwave.
Never had discoloration, but there was one time the water turned red for a Iron Man figure. The metallic red paint on the IM figure wasn't affected.
Turns out there was some glob of paint somewhere that never cured.
And honestly, the hotwater/freezing method has always worked for me. I've only ever had to use a blade just to cut away sculpt to increase motino from the joints. hot water helps in softening the plastic, so it's easier to cut and you make fewer mistakes that way.
Just to note, i also wash my figures to keep them from attracting dust. BAsically all toys have a thin layer of lube from the production/packing process and this lube makes dust stick on easier and harder too. Washing them makes future dusting a breeze, especially if you use canned air.