I'm assuming that what you want is to simply select the color of the shade so that you can use it elsewhere, not that you want to change the shade itself.
There's an easy way to do this if you're working with transparencies & a 3D shape that are producing your desired color. If you're working in Illustrator, all you really need to do is make a copy of your object (to preserve your work) and select all of the related objects.
From there you just go into the menu bar at the top, select the menu labeled "Object" and then click "Expand Appearance". Repeat those last few steps again and from the Object menu click "Flatten Transparency...". A menu will show up, you should be fine to just hit ok with the default options, if you're just doing this to select a color. The end result should be paths that have fills of the colors displayed on your screen, not transparencies.