Similar to how Minecraft allows you to select the block your cursor is over via middle mouse click,
wouldn't it be intuitive to make the eye dropper/color selector bound to the middle mouse button?
As far as I can tell, a middle mouse click does nothing in PDN right now and when I tested to see if this feature was already implemented, it just felt right to me.
So I say follow in Notch's footsteps with the middle mouse button, afterall he himself advocated PDN on his blog for creating games