There is a colorpicker in WordPress 3.5. It's cool that you use CSS3 gradients and it's easy to be created but I've got a few suggestions:
1) Microsoft Paint, GIMP or Adobe Photoshop - their default colorpicker mode is better for average person. HLS - not good. In this case you should use what top corporations have used as default settings for great products. I know that HLS is more for designers but RGB is more for average human. Anyway, having an option to pick one of those modes would be the best (either by an user or by plugin/theme developer).
2) 8 color presets below picker - honestly, that doesn't make any sense. It's absolutely redundant.
3) Many WordPress modules need RGBA() version of a color. This is crucial. Alpha is often important.
4) Having input field show HEX or RGBA value of a color all the time = highly recommended. Why do users need 2+ clicks to access that if they can do that with 1 click? The correct way to do this is to have TEXT INPUT and inside that INPUT a square with the color (perhaps to the right). Currently you only have a square and no input field unless user clicks on a square.
5) Maybe no message "pick a color" at all? It is quite clear anyway.
6) Last but not least - currently WP3.5 colorpicker supports only position relative (shows picker below INPUT and alters height of the container structure) - how about a mode where picker has position absolute? It's again - Photoshop style.
7) API? Settings? Methods? Events? Where are they?
Well, to conclude: I would say that it's best to check how Photoshop's color picker works and copy that. It would be great to have all these improvements :)