well, I've never tried to apply a filter with a brush, because I would think that a few brush strokes in it would kinda ruin the effect... the idea is to get a uniform change in color over the entire model.
As to how subtle the change can be... here's the only example I have a picture of... using a filter of VMC Transparent Green and VGC Thinner Medium (1-3 ratio)
Before :
After:
I've also used
GW Badab Black straight up, if a model turns out a bit to "bright green", and I want to tone the whole thing down a shade or two. A very light coat is all it takes, but its a pretty subtle effect. I don't have any before and after pictures of using it though.