The trick would be using a transparent glaze, which would allow the decal colors to show through without obscuring them. If I were going to try doing this I'd use a mix of Vallejo's transparent green & smoke; AK Interactive and Tamiya both make equivalents I think.
Dutch
(I'm a landscape painter, and artists have been referring to this technique as "glazing" for the past couple of centuries. I see modelers call it "filtering" nowadays :-)