Gnome desktop loves to remove things. The option in the GUI settings to have a Color is yet, another removal.
You can still use the terminal (as always) to set things how you want:
This sets the desktop to have No Picture:
gsettings set org.gnome.desktop.background picture-uri ""
Use a color picker to choose a color you want and get the RGB or the HEX for it.
Replace the HEX code below with the color you choose in your terminal command:
gsettings set org.gnome.desktop.background primary-color '#474747'
Alternatively, if you still prefer a GUI method- you can use dconf-editor
sudo apt install dconf-editor
Launch from app menu or alt+F2
Navigate to /org/gnome/desktop/background/primary-color