A few suggestions

[Traik]

From the most important to me to the least important:

1. Having alpha or rgba() would make it complete.
2. I'm not sure if this is possible or not but absolute positioning of color picker would be a cool improvement (so that it can float above elements like in Photoshop).
3. Maybe no message "pick a color" at all? It is quite clear anyway. I would add an alternative mode where upon initialization it still shows old input field and to the right of that input field you see a solid little square - exactly like this person did: http://dribbble.com/shots/232627-Custom-colour-picker (it's probably Iris).
4. Additional controls (at least as an option) where user can manually put R, G, B or H, L, S numbers is sometimes useful to me in Photoshop as well.

Basically, I think that companies like Adobe or Autodesk deliver nice colorpickers in their products that can be used for further improvements. And for a person that is half-wordpress-user-half-designer (like a person setting up unified brand color for entire WordPress website through admin panel) some more options like number (4) might be useful.

I hope that some of my ideas will help make it better :)

[mattwiebe]

Having alpha or rgba() would make it complete

See #13

I'm not sure if this is possible or not but absolute positioning of color picker would be a cool improvement (so that it can float above elements like in Photoshop).

See #9

Maybe no message "pick a color" at all? It is quite clear anyway.

That's not Iris. That's the wpColorPicker implementation in WordPress that uses Iris and adds a bit more.

Additional controls (at least as an option) where user can manually put R, G, B or H, L, S numbers is sometimes useful to me in Photoshop as well.

Iris is specifically intended to have as few controls as possible. I'll leave the profusion of fields to people who like to have confused users.