Based on their sound alone, theGreencards deserve instant citizenship. This trio of English and Australian expats (Carol Young, Kym Warner and Eamon McLoughlin) pursues the folk-friendly, acoustic sound of Alison Krauss with ears cocked to American soft rock and neo-bluegrass. The Greencards' newest album, Viridian, is their best: Lead singer and bassist Young phrases with jazzy playfulness, principal songwriter Warner pushes his mandolin into David Grisman territory, and McLoughlin (when he isn't busting a hot break or reinterpreting A.E. Housman stanzas) meticulously overdubs violin, viola and cello for intricate orchestral effects. Together they prove that sometimes it's outsiders who can get all the way inside and under the Americana skin.