I've just upgraded to Ubuntu 20.04, and there's no obvious way in the settings to get rid of desktop wallpaper, if all you want is a plain background. There's a quick way at the command-line though:
gsettings set org.gnome.desktop.background picture-uri none gsettings set org.gnome.desktop.background primary-color '#FF0000' gsettings set org.gnome.desktop.background color-shading-type 'solid'
The first line turns off the wallpaper. The second line sets the color to red. (You can change #FF0000 to the hex code for the colour of your choice, or you can use the colour names like 'red', 'orange', 'blue', etc from the X11 Color Names list). The last line sets the colour to be a single, solid colour.
You can also change color-shading-type to either horizontal or vertical to get a gradient. To specify the second colour, you add:
gsettings set org.gnome.desktop.background secondary-color '#00FF00'
Quick note on gsettings
You can see all the options under org.gnome.desktop.background with:
gsettings list-keys org.gnome.desktop.background
and for a quick description of the keys you can use, for example:
gsettings describe org.gnome.desktop.background primary-color