If you want to restrict the color picker to only show you Web-safe colors, you may do so by checking the "Only Web Colors" option in the bottom left corner of the color picker. This restricts the color picker to only displaying colors in the Web-safe palette.
Color Libraries Button
You'll also see a color libraries button below the OK and Cancel buttons in the color picker. This is used by professional designers who need to use specific brand-name color libraries for process printing.
Picking Colors from Other Images and the DesktopYou can also pick colors from an open image or anywhere else on the screen while the color picker is open in Photoshop. To pick colors from an image you have open in Photoshop, simply move the cursor outside of the color picker. As it moves over an open image, the cursor will change to an eyedropper and you can click to jump to that color in the color picker.
To pick a color from a part of your screen outside of Photoshop, move your cursor outside of the color picker and onto an open image, just like I described above. But instead of clicking and releasing the mouse button, click and keep the mouse button held down. While keeping the mouse button held down, move the cursor outside of the Photoshop workspace to an area of the desktop. As you move over the desktop, you'll see the new color box in the color palette change to whatever color is under the eyedropper cursor. When you're hovered over a color you want to use in Photoshop, release the mouse button and the color picker will jump to that color.