In the wee hours...I figured it out...I think. ...That is, I'm hoping it's not a fickle twist of luck!
I "unselected" "use layer colors" in the document preferences and, lo and behold, my class colors returned!! I then rechecked "use layer colors" so that I could fiddle with my class colors in viewports. The result, 2 identical viewports with 2 different color schemes to illustrate 2 phases of work. To add more classes that have color attributes, I suppose I will have to uncheck "use layer colors" again.
Many thanks for coming to the rescue!!