Disclaimer - I'm not an expert, and I've only started playing with filters on my last few kits, but my understanding:
- A filter is a kind of wash. Thinner than a wash you'd run into panel lines.
- A filter is meant to be applied over the entire vehicle
- The wash overlays a basically transparent color on top of the paint scheme, decals, etc, providing a very slight shift in overall coloration.
- The wash reduces jarring contrast (too-vivid decals, too-contrasty camoflage, etc), since the very thin coloration, while subtle, kind of ties everything together.
As far as Mig filters? I think they're too expensive when artist oils and Mona Lisa odorless thinner can do a better job (I like the way oils perform for this) for a much better value (since a tube of oil will last far longer).
I filtered this P-47 with Winton & Newton raw umber heavily thinned with ML odorless thinner:
On the Bench: 1/32 Trumpeter P-47 | 1/32 Hasegawa Bf 109G | 1/144 Eduard MiG-21MF x2
On Deck: 1/350 HMS Dreadnought
Blog/Completed Builds: doogsmodels.com