There is one tool that every designer on a Mac should use and that is the DigitalColor Meter. If you look in Utilities > DigitalColor Meter, you will find this awesome app.
Its very simple to use. What it does is you your mouse point as an eyedropper, similar to the ones used in Photoshop etc. One the left you have a small mini view of what you are capturing, this is in the form of a small square. You can change the aperture size, a big aperture will take an average colour value, a small one will be more precise.