Holding down Shift while clicking multipul times with the eyedropper will have an "additive" effect. In other words, if you held down the shift while clicking diffrent greens in your shot, you will eventually have ALL the diffrent shades of green selected then you can use the slider to fine tune.

Hope thats the part your missing.

Let me know if you have any other questions....or this dosnt answer your current one.

Roman