Yes, there is a possibility that your monitor is defective, but since creating a new user account temporarily fixes it, the root cause is highly likely a corrupt software configuration or a stuck macOS system preference.
Hardware defects (like a physically faulty panel) do not care which user account is logged in. If the problem completely disappears on a brand-new user profile, your hardware is physically capable of producing a perfect picture, pointing directly to a software glitch or a corrupted cache file in your original user profile.
However, the iMac's LCD technology can also experience temporary image retention (ghost screen) caused by electrical or thermal stress on the pixels. When combined with a software bug that alters color profiles, it can make the screen look severely faded and washed out.
Why this happens and how to fix your main account
The fact that it happened after an update or suddenly points to a corrupt color profile or a corrupted Accessibility configuration file. Try these exact steps on your original user profile to fix it permanently:
Check Accessibility Contrast Settings
Sometimes macOS randomly glitches and bumps up the display contrast, which makes videos look incredibly washed out and white levels look entirely blown out.
- Go to System Settings > Accessibility > Display.
- Ensure that Display contrast is set exactly to Normal (the slider should be all the way to the left).
- Toggle Increase contrast on and off to reset the display buffer.
Reset the Color Profile
Your original user account might be loading a corrupted display calibration file.
- Go to System Settings > Displays.
- Look at the Color Profile drop-down menu. If it is set to a custom or calibrated profile, switch it back to the default iMac or Display P3 profile.
- Trick: Some users clear this specific glitch by turning Invert Colors (in Accessibility settings) on and back off, which forces the system to recalculate the color lookup tables.
Clear System Caches via Safe Mode
If there is a corrupt display cache in your system files, you can flush it out using Safe Mode.
- Shut down your iMac completely.
- Press and hold the Power button until you see "Loading startup options" on the screen.
- Select your main hard drive, then press and hold the Shift key and click Continue in Safe Mode.
- Log into your main account while in Safe Mode, let it sit for a minute, and then restart your Mac normally. This clears system caches that could be carrying over the color glitch.
How to test if it actually is a hardware defect
If the ghosting and fading come back on both user accounts and nothing fixes it, it is a hardware issue.
- The Ghosting Screen Test: To clear real LCD image retention, set your screen saver to a fast-moving, colorful option, or play a full-screen, high-definition video on YouTube for about an hour. If the "ghost images" completely disappear, your panel is fine, and the liquid crystals just needed to reset.
- Hardware Evaluation: Because the iMac M4 is a recently released machine, it may be fully covered under Apple's 1-year limited warranty. If the issue keeps coming back across all accounts, back up your data and bring it to an authorized provider to have the display panel or logic board tested for a hardware voltage defect. Even if your Mac is out of warranty and you do not have AppleCare+, you can visit a Genius Bar for a free evaluation.
Genius Bar Reservation and Apple Support Options - Apple
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