It turns out that finding a Flex 3 color wheel component is pretty tough. I came across some classes written by Mario Klingemann and Ryan Taylor that looked promising and so here I've put together a component which uses these.
See test/ItsLikeACircle.mxml for example usage. The short version is you can
listen for change (technically ColorPickerEvent.CHANGE) and close
(technically Event.CLOSE) which do the usual things. close is dispatched
on MouseEvent.ROLL_OUT and change any time the color changes.
So to try this out simply run, from within the project root, test/build.sh
(make sure build.sh is +x'd). This will put ItsLikeACircle.swf into
test where index.html already exists. Open index.html in your browser
and you should see the simple example.
Please feel free to fork this or submit a patch or whatever. Also feel free to report serious bugs in the Issue Tracker here on GitHub.