little butterfly

A few days back Isa caught this small butterfly in the lawn. I try to encourage her just to catch the white cabbage moths, and leave the others alone! O well. She put it in a jar with holes poked in the lid, and some plucked flowers she'd seen it feeding on, and it survived for several days (great for observation).

The colors have quite faded now that it's dead- it's actually more green than yellow.

I have no idea what kind it is. Does anyone recognize it?