It's flight is a flicker of green and brown mingled, until it settles with closed wings and the green under-wing is revealed.
In the sequence below the green hairstreak has settled in the grass and you can see the ochre colour on wings more clearly.
'The GH has green fore and hind wings when at rest ......ie with its wings shut. [It never opens its wings at rest as the camouflage against a leaf is so important.] However in flight the green is hardly noticeable.....instead it looks brown from the inside colour of both sets of wings.
If you see one always follow it and others are often tempted to 'buzz' it in flight......they are very colonial.;
Thanks to Chris from Cumbria Butterfly Conservation'