Illustrator has a number of different color pickers that appear depending on what particular item you click on, but the one in the color panel is the least useful and most tiring to use.

As far as I can tell, this picker is the Brightness Cube from Photoshop but without the brightness slider that’s needed to make effective use of it. If you use the sliders to adjust saturation or brightness, the change isn’t reflected in the palette square.

With this picker, you can either reduce saturation but with maximum brightness, OR you can reduce brightness but with maximum saturation. So your only choice is to select a color vaguely similar to the one you want and then fiddle with at least 2 different sliders to achieve your desired color. You can't even select a shade of gray without going through this process.

Just copy Photoshop’s color panel. If nothing else, just use Photoshop’s Hue Cube color picker, which is identical to the color picker already used in most places in Illustrator.

As it stands, I have to spend $30 on the MagicPicker plugin just to get a decent color picker in my sidebar.