It’s a bug in 10.13, and word is Apple is aware of the problem and is looking at fixing it (no custom color pickers are loading at all on the GM release, which is a huge huge bug )
At any rate, there is a way to get back a hex color picker, if not the same one as before.
Open up the Color Panel, and click on the sliders tab.
In the palette that comes up, selected the “RGB Sliders” option if it isn’t already.
Then, a little hex field will show up, where you can enter your RGB hex values.
Hope that helps,