custom wallpaper solid color PROBLEM

Known issue:

Custom wallpaper color shows as white and… - Apple Community

Or, rather than my Preview method, use any app to show the color you want, press Command+Shift+4, and clip out a tiny square of the color. Put the PNG file it saves in your user account Pictures folder and then choose that as your desktop image. It will automatically tile to fill.

Will make it easy for you. Download this pic.

Welcome. Might be awhile. Certainly not until 15.5 or later.

Irritating indeed if you open the computer after a major update, can hardly read your screen, and need to start looking for fixes!

The work-around is to create a jpg picture with the screen color your want and select it in the wallpaper settings. That at least works.