Here is how to do it:
Step 1: From the ‘Finder’ menu, choose ‘GO->Utilities’ or click Shift + Command + U

Step 2: Next, open the app, ‘Digital Color Meter’

Now, you can change the aperture size to smaller (left) or larger (right). Also, you can use the drop down menu to choose what format you want it represented in. For HTML coding, use RGB As Hex Value, 8-bit.
To copy the displayed color value to the clipboard, hover the mouse cursor over the color you want to measure and press Shift+Command+C. Your needs may be different, and it gives you several options to choose from.

digitalcolor meter keyboard shortcuts:

• Lock Position (Command+L)
• Lock X (Command+X)
• Lock Y (Command+Y)
• Copy Image (Command+C)
• Save as TIFF (Command+S)
• Hold Color (Shift+Command+H)
• Copy Color As Text (Shift+Command+C)
• Copy Color As Image (Option+Command+C)

More about DigitalColor Meter on Wikipedia

Digital Color Meter for Web Designer is a post from: My MacOS X