Hello MurPhil,
Apple Support Communities is a great place for help with the display on your Mac. It sounds as though you’re on the right track for addressing this issue as well. When going in to System Preferences > Accessibility, are you able to click Display in the left column? If so, does Display contrast appear there? If so, are you able to adjust the contrast at that time?
If the message doesn’t show immediately upon powering on the Mac, does it appear after you log in to your user account? If so, does it happen if you sign in to a different user account on your Mac? If you don’t have an alternate user setup, you can create a test user following the steps in the article below.
Set up users, guests, and groups on Mac
Let us know how that goes!