Color off in second monitor

Here is a new problem. The color and darkness in my second monitor is varying on its own.

I have a Mac Mini running OS 11.2.3. The primary monitor is plugged into the HDMI port. There is no problem there.

The second monitor is an old VGA monitor. It is plugged into one of the Thunderbolt-USB4 ports.

Yesterday and today this VGA monitor has fluctuated in its appearance. Now the picture in the monitor is not as bright as I would like, and it has an dusky tint to it, as if the monitor was set for a room with dim light.

I looked in the Display preference. I did not see any useful calibration tool there.

How do I get this secondary VGA monitor to look the way I had it day before yesterday?