Cannot calibrate Retina iMac when booted from external Mojave drive

[steve...]

2015 iMac currently running Mojave on the internal drive. The retina display looks great. But when I boot from an external ssd drive also running Mojave, I cannot get a good image on the retina display.

Tried calibrating with system preferences > displays > color > calibrate and also Spyder Pro color calibrator. Nothing seems to help. What caught my attention was the faint lines separating the days in Calendar and also the subtle gray/white backgroud lines here on the forum. When booted from the external, they're all washed out. Any ideas? TIA

[PeterW]

Are all the Display prefs the same in both copies of the OS?

[steve...]

Yes, as far as I can tell, they're the same. I even copied the profile over with no change. I had an external Monterey drive running for a few years with no problems. My plan was to upgrade the internal to Monterey when the external Mojave drive was ready.