Sorry, bad English !!! (Turkish)
Zorin OS 15.x Core background settings:
Menu->Settings->Appereance->Background-->(Wallpapers/Pictures/Colors)
No problem ! (Use Colors)
Zorin OS 16 Beta Core background settings:
Menu->Settings->Background-->(Only Wallpapers)
Problem !!! Can't use flat colors.
???
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.
https://www.google.com/search?channel=fs&client=ubuntu&q=color+picker
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 dconf-editor
Navigate to /org/gnome/desktop/background/primary-color