The above photo shows six models painted identically in fast, bland colours - no shading, highlights or layering. But two of them have been treated with Dark Tone: can you tell which, children?
When you are churning out forty ranked axe-dwarfs for a regiment this is a Godsend.
I don't dip the models but paint the varnish on, thinning with white spirit as necessary. However careful you are the lid will clog up and so expect to throw away about half. The pigment sinks to the bottom of the varnish but can be lifted up by the brush. I use cheap children's paint brushes.