Users on OS X 10.10 may experience an issue where their color picker does not display the zoom lense graphic allowing easy selection of an existing color onscreen by pixel.
While this issue has already been submitted directly to Apple, in the meantime, it is possible to use the tool without the lense UI functioning. Simply click the eyedropper icon in the Colors window (rather than the magnifying glass in previous OS versions) and then click the desired color, it should be brought into the color picker as expected.
Magnifying glass UI on 10.9:
Lack of magnifying glass UI on 10.10:
Edited by JimW