Here are a few shortcuts for using the Eyedropper tool in Photoshop:
• The Eyedropper’s Sample Size- set in the options bar – affects the Magic Wand, Magic Eraser and the Background Eraser.
• Option + (Mac) | Alt + (Win) -click the Eyedropper to select the background color swatch (instead of the foreground) in the Tool panel.
• The Eyedropper can sample colors from outside of Photoshop. Make the color visible (on the desktop, in another application etc.). Then, click with the Eyedropper within the image area and then drag to sample the desired color on the desktop/other application.