Male Brimstone butterflies. The male and the female look similar from the side and rarely rest with wings open. They are perfectly camouflaged as leaf-lookalikes.
He has a more yellow tinge to the green undersides and is bright sulphur yellow when he opens his wings.
The female is paler from topside and underside, looking almost white from the top, but distinguishable by the leaf-shaped wings.
The male has a yellower-/greener- underside.
Brimstones are often the earliest butterflies to be seen in the year since they are one of the longest-lived (12 months+) and hibernate as adults.
The buttery yellow colour of the male's wings in flight gave rise to the word "butter-fly". (Or was it from old English for "beautiful fly"?)