Depends on your CSS, but the ColorPicker control definitely uses a Button in its implementation. Guess they missed the styling issue, but there's a reason for moving (partially) to the Spark library.
I agree with moving to the spark library and I use many of the spark components in my application. But there are places where mx buttons are needed.
Like the Alert object. So I have to skin the mx button. On the other hand, there isnt a Spark ColorPicker either.
and by the way, the css for the mx button is this
so I didnt find a solution to the problem. I've decided not to waste more time so I've deleted the css for the mx button and everything is fine.