You just mix paint, say light grey, with the varnish, say 5%, then airbrush it on. the original colour is never obliterated cos the varnish is transparent so you can blast away till your hearts content. Mat varnish has the same effect but to a lesser degree and you may not want a dead matt finish. Satin varnish seems to work best, it has a unifying effect which can be quite subtle, the secret is to mess around on something that doesn't matter and see what happens.
Haddock