Can I use dilute Vallejo Model color sufficiently to use it as a filter?

I have, in the past, mixed Vallejo's model air very thin and sprayed it on as a 'filter' effect, i.e. exhaust trails or carbon residue.  I've also mixed model air with varnish (clear coat) to dilute the pigment, and with mediums to dilute the pigment.  I've had more success with the varnish mix, as I find the mediums a little thick when sprayed.  This being said, I do this with an airbrush.  I've never experimented with brush painting, if that is your intention.  

Personally, I use about ten drops of thinner or varnish to one drop of color.  I then thin the mixture to spray.  In this way, I'm only getting a small amount of color deposited, which is what I'm aiming for in a filter.  

Model color is a LOT thicker than model air.  I can, though I don't often, spray model air straight from the bottle (I usually thin it 30%).  Model color has to be thinned at least 50%, if not more, to spray properly, in my experience.  

In the end, I guess I will say I have done it with a line of their paints, not the line you are using, so take from this what you will and experiment.

Lol!  Now you know why I don't work in 1/72 anymore!

Looks like CN misread 'filter' as 'filler.' I do that all the time myself. :-)

Diluting with water to filter consistency will be problematic due to the high surface tension of the water. It will bead up and create tide marks.

I suggest you experiment with Vallejo's model washes or Game Color inks, they are basically transparent paints that you could probably dilute without using so much water.  You could also try Vallejo's

mediums, such as thinner medium, matte medium, etc, to mix your own transparent paints.

In my experience, it's a bad idea to try and use paints as fillers.  They continue to dry and shrink for quite a while so even though the seam looks good today it might crack in a week.

Vallejo paints are water based acrylics. Spray a coat of clearcoat before using washes.