iMac Pink Screen and Restart Upon Awakening

Yes, the display issue you mentioned is relevant.

Apple's repair documentation for the 24-inch M1 iMac specifically covers both Display and image issues and Intermittent shutdown or system instability. Apple's 24-inch M1 iMac repair manual — four ports Apple's 24-inch M1 iMac repair manual — two ports

Your sequence, wake from sleep, enter the password, the desktop appears, then a few seconds later the display turns pink and the Mac restarts, could involve the display/graphics system during the wake process, but it could also have another software or hardware cause. 

Apple identifies both software and hardware as possible causes of unexpected restarts. Apple: If your Mac restarted because of a problem

Before making further changes, make sure you have a current backup. 

Then check System Settings >> Privacy & Security >> Analytics & Improvements >> Analytics Data for a panic-full report corresponding to one of the pink-screen restarts. 

If one exists, its contents may provide evidence about what triggered the restart.

Also note exactly what happens to the display immediately after waking. 

If it is already faint, washed out, flickering, or otherwise abnormal before the pink screen appears, that is important information because it may help establish whether the display symptom and restart are related.

I would not yet conclude that the display hardware is defective, nor that it is responsible for the failed macOS 15.8 installation. 

The failed installation may have its own cause, although an underlying restart or system problem could potentially interfere with an update. 

I would therefore avoid repeatedly attempting the upgrade until the restart problem has been investigated.

If the panic report does not clarify the problem, the next controlled tests are to disconnect unnecessary peripherals, test in Safe Mode, and run Apple Diagnostics. 

Apple recommends these steps when investigating repeated unexpected restarts. Apple: If your Mac restarted because of a problem

Because this is an M-Class iMac, Safe Mode and Apple Diagnostics are accessed through the Apple-silicon startup options. Apple: Start up your Mac in macOS Recovery

The most useful information to obtain now is the panic report plus a precise description of the faint-display symptom. 

Those two pieces of information should help determine whether this is primarily a display/graphics issue, a wake-related software problem, or something else.

Welcome and thank you for reaching out. 🤗

If you have any antivirus software installed, then remove it A.S.A.P. as they interfere with the system operations and performance.

If you don’t have anything like that installed, then try the following:

First, let’s see if this issue happens in Safe Mode:

1. Shut down your Mac.
2. Press and hold the power button on your Mac. As you continue to hold the power button, your Mac turns on and loads startup options. When you see Options, release the power button.
3. Select your startup disk. It's named Macintosh HD, unless you changed its name.
4. Press and hold the Shift key, then click the “Continue in Safe Mode” button below your startup disk.
5. Log in to your Mac. You might be asked to log in again.

If the strange behaviors do not occur, then it may possibly be a software issue.

If the issue doesn't continue in safe mode:

Leave safe mode by restarting your Mac normally, then try to reproduce the issue.

If the issue doesn't return, it might have been resolved by the other things that safe mode did at startup, such as checking your disk and clearing caches.

If the problem persists, then let’s try this:

• Software Update.
• Use Apple Diagnostics to test your Mac.

Need more help?

• United States: 1-800-APLCARE.
• Contact Support.
• Mac Support.
• Mac User Guide.

Good luck! 👋🏼😉

Smiliñ 😎 Brian | Community Volunteer

Before I had a chance to go through your great info I got sidetracked. I disconnected my external disk drive and all the pink screens and restarts stopped. Everything works fine except when I tried to upgrade from OS 15.7.3 to 15.8-- pink screen, restart. Perhaps the drive was robbing too much power on wakeup? The attempt to upgrade was the same, after about a minute of loading it failed...