I think different people use different brands, but I use a Deco Art antiquing medium and blend in whichever paint I’m using. I use Golden paints on the Windstones (which is the brand Windstone uses) but have done the same with craft brand paints on non Windstone figures. No reason behind the Deco Art, it is just the antiquing medium I was able to find. It has worked fine for me though.
I use a blob of the antiquing, just enough paint to color it, and a dab of retarder to be safe. Smear it over a fairly small area making sure you get it down into the cracks and crevices, then wipe it gently off before it can start to dry (that’s why you work in small areas). I also clearcoat the figure before antiquing so I don’t so easily damage the paint job in wiping or by the paint being stained by the antiquing,