Making a coloured screen

Desktop wallpaper occupies one level, so you can have an image or solid color, but not both, unless you create new wallpaper using your favorite graphics application and layer the image on a solid color background layer and then save that for use as wallpaper.

System Preferences >> Desktop & Screen Saver.

In Desktop tab >> Colours and choose a Solid Color of your choice

You are welcome. Ensure that the final artwork is sufficiently large for your screen size, and with adequate DPI to prevent jaggies around your image.

Sorry. Already been answered.