When in the CSS Editor mode and the “Colors” panel is open, click the magnifying glass icon to select a new color based on any part of the screen. It’ll bring up a bigger magnifying glass that, upon mouse click, will load the selected color into the “Colors” swatch.

Most think that this icon should be a color picker (and I agree), but as Panic’s Cabel notes, “This is actually a feature of the system-wide color picker in Mac OS X! You’ll get the same feature if you open, say, the color picker in Text Edit or Mail.”