Green Hairstreak, Callophrys rubi.
Green Hairstreaks are brown on the upper side of their wings, the side that is only seen in flight as these butterflies invariably perch with their wings closed, displaying the vivid green underside you see above.
The iridescent green is not comprised of colour pigment. It is, in fact, structural colour, caused by interference effects. For more on iridescence, you might care to click here.