I think a wash and a filter serve different purposes, whatever the niceties of the definitions. I consider them different things.I usually apply a wash of one sort of another to a model to bring out detail, fasteners, access hatches etc but without overdoing it. For me the results are hardly noticeable for being there, but you'd notice if they weren't they (if you know what I mean).
A filter is an entirely different thing and is a way of altering our perception of the finish. I've used multi-coloured oils on some models, like an all black P-47, but generally now use a slightly tinted gloss coat over the
decals which serves the same purpose.
I find that a grey tint will help to reduce the overall contrast between camouflage colours, give a very slightly aged look to the paint and help to blend in
decals all in one go. An added bonus is that it is very simple to make and apply. I just add a drop or two of a grey acrylic paint (I find the green/grey RLM 02 or RLM 75 work well) to my gloss varnish. Don't overdo it! The effect is quite subtle.
Yesterday:
Today with filter
I'm not sure that my photographic skills are up to showing a subtle difference, but it is visible to a Mk1 eyeball
Cheers
Steve