There may very well be a difference with the OS you mentioned, but try this: Start with the Apple menu ... then System Preferences ... now Desktop and Screen Saver for available choices. Click the Desktop option ... you will then see a window on the left containing various folders with choices ... small previews will appear on the right side.
You can change your current desktop to one of the others by clicking on another thumbnail image ... if a solid color is your preference, choose Solid Colors from that left side list and pick the one you want. When satisfied with your choice, close the Preferences window.