Use the magnify tool to capture colors on the screen. To bring up the magnify panel press ⌘F.
Hover over the portion of the screen with the color or colors you want to capture.
Press control+option+command+L (⌃⌥⌘L) simultaneously to capture the portion of the screen.
When the magnify image is locked, hover over any of the colors and click on the color you want. Now you can press the button on the bottom to send the color to the palette. You may also drag the selected magnify color onto any color well or text.
Note: If you can't remember the key combination to capture the screen, just look at the menubar under Tools > Lock magnify image.