Ah, so hard to explain it in a way that makes sense. Saw a guy in the armor forum building a great KV, explaining the filter technique, but explaing it in a way that made no sense.
Check this link:
http://www.missing-lynx.com/rareworld.htm
There is an article there on filters. Its basically about as good of an explanation as you're likely to find. FIlters are VERY VERY VERY thin washes. IMO, water based filters work the best because the water gets tinted. Oil based filters are very difficult to get right, because the pigment always seems to settle as separate matter than the thinner used.
THese washes are applied with a large, wide brush, over the ENTIRE model at the same time. It helps to blend it different elements of the vehicle together, particularily camo schemes. Filters should be applied probably immediately after paint chipping, camo painting, and other basic steps that follow the basecoat. This technique will allow you to UNIFY all of these elements, and allow further weathering to look more realistic.