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.
One the right you have the colour values. There re various options you can chose from using the drop down menu. If you pick RBG as Hex, you get the format used in websites and photoshop. Clicking on the different ones brings up different options. Experiment.
There a millions of uses for this little program. No longer do you have to take a screen shot, you can directly pick off the colour value. Its great for design if you need to find out a background colour or any other colour in a design. It is especially useful if you need to match a colour when you are designing a website.