I don't think it's actually part of the eyedropper selection tool anymore. It's been gone for awhile. When you click a color in a clip, that color that is selected is displayed (at full saturation) and all others are desaturated and lightened — it's fairly easy to tell what has been selected. Hold down the shift key to add another hue — or drag carefully.
Best practice* (for the 3D color mask), select one end of the color range (or a color nearest the most important edge of the selection). Hold the shift key down and click on the other end of the color range. Then try to fill in the rest with as few clicks as possible. (I generally do not drag the selection anymore.)
Best practice for the HSL color mask: Click once. Work the slider controls to get everything else. HSL takes practice! It also doesn't seem to work as well when dragging selections.
[*in my experience...]
Sometimes it helps to turn on View Masks to make sure you've gotten everything you want (or especially if you've gotten more than you want).