In the CSS Pane's Rules view, if a rule contains a color value, you'll see a sample of the color next to the value:
If you click on the color sample, you'll see a color picker popup, enabling you to change the color:
The color picker includes an eyedropper icon: clicking this icon enables you to use the eyedropper to select a new color for the element from the page:
Starting in Firefox 40, clicking the color sample while holding down the Shift key will change the color format: