Colors are suddenly inverted on my iMac. Clicking color inverting box in accessibility does not help. Also, an "activity monitor" box appears on start-up. Why is that?

Welcome to the Apple Support Communities!

I understand you are seeing inverted colors on your iMac, and the Activity Monitor application when you start your iMac. To address these issues, first shut down the computer. Then start it in safe mode (holding down the Shift key) to see if you can isolate the issue.
Use safe mode to isolate issues with your Mac

Start up in safe mode

To start up in safe mode:

1. Start or restart your Mac, then immediately press and hold the Shift key. The Apple logo appears on your display. If you don't see the Apple logo, learn what to do.
2. Release the Shift key when you see the login window. If your startup disk is encrypted with FileVault, you might be asked to log in twice: once to unlock the startup disk, and again to log in to the Finder.

To leave safe mode, restart your Mac without pressing any keys during startup.

If an issue doesn't happen in safe mode

If an issue doesn't happen when your Mac is started in safe mode, try restarting without pressing any keys.

• If the issue comes back, you might have incompatible login items.
• If the issue doesn't come back, it was probably caused by a cache or directory issue with your startup disk, which safe mode fixed.

If all the colors on your screen are suddenly inverted or black and white, you might have pressed Control-Option-Command-8, which is a Mac accessibility shortcut for inverting the screen colors.

You can control this feature in Accessibility preferences:

1. Choose Apple menu () > System Preferences, then click Accessibility.
2. In the Display section, deselect "Invert colors" or "Use grayscale."