Shoot enough pics of Brimstones flitting between flowers and you will eventually get some upper-surface shots, not normally seen because they always settle with wings closed. Here's a study on their colouring:
Male. The upper forewing is more deeply coloured than the rear.
In reflected light female (left) and male (right) appear rather similar.
Males appear a deeper yellow when the light is transmitted through those two upper layers:
Females appear more green in transmitted light.
All shot in West Wood today.