Photoshop: Eyedropper - things you did not know

To choose foreground colour from an image, use the eyedropper tool (I) and just click on a part with the desired colour. To choose background colour from an image use the eyedropper tool (I) and alt-click on the desired colour.

To select a colour on the screen outside an image: click and hold down the colour picker and drag it to the point on the screen with the desired colour.

To swap foreground and background colours: press X. To set the default foreground and background colours to black and white: press D.