Colors is a simple application for anyone who needs to be able to pull colors from what you can see on the screen. Colors takes Apple’s basic color chooser (complete with “magnifying glass” to pull colors from other applications), and combines it with an easy way to copy common formats of color values (eg., #rrggbb, rgba(r, g, b, a), etc.) to the clipboard. Colors is free of cost, but not open source.
Grab it from Matt Patenaude site.