his taking a screenshot won't make any difference cause if the highlighting truly isn't working, then no color will show
That's a problem with the System screenshot function, if you use command + Shift + 4. While is does show a selection, it is normally light grey. However, using command + shift + 3 gives you a screen shot that includes selection highlighting. Granted, cropping out 97% of that image takes just a bit of work, I think it would be worth trying to make a reply. I think even
Mojave macOS 10.14 had a "Screenshots" display with descriptions of what each keyboard shortcuts do. And there are so many free/cheap screen capture apps out there, why bother with OS limitations?
Actually, I suspect his problem may be related to his contrast, saturation settings. Likewise, when you pick a different color "dot", a white dot appears in the center of that choice. Is that white center what is confusing him?
I don't mind if he found another solution from some place else. We(I) might just learn something.