The Brimstone is usually the first butterfly to appear in the spring. However, in the first few weeks they are usually flying from A to B all over the park, and never seem to stop. It took me two months to finally see some stop and feed on flowers. The female (below) is green-white in colour.

The male (below) has sulphur-yellow wings, which gives the brimstone its name. Brimstones always feed with their wings shut, so you are unlikely to see the top of their wings. Don't worry, the top looks very similar to the underside, so you're not missing anything.