Rainbow Screen (not spinning circle)

Unfortunately, it looks like the graphics processor on your iMac is failing ... or possibly the screen itself is faulty. At this point, you can try the following to see if it can help: Mac Basic Startup Troubleshooting Steps - Apple Community

Try steps 2, 4, 5, 8, and finally 10.

I have a mid 2010 iMac running OS High Sierra ( It will not update to OS Mohave . But in the past (?) two Months I have experienced very Bizzare Screens ( My iMac gets > Vertical Stripes ( each about one inch wide - alternating purple Or green with gray/ white stripes with " blinking pixel like icons . I did use every combo key restart / usually > safe boot restart ( Shift key hold > power on until start up bar line progresses . I agree with prior posts something is wrong - ? iMac last release - 2017 unsure when You purchased Yours (?)

I am considering buying a iMac though it is 2019 and model available = 2017 ( Perhaps a laptop this time for me )

Best

diamondblues

The report does indicate that your Mac has limited amount of RAM and has been swapping to the drive when running multiple programs. You should consider increasing the RAM amount or reduce the number of apps running concurrently.

The other item to check it under Configuration Profiles in the report. It may be that PIA (I'm assuming that is the Private Internet Access VPN) may have added this. However, I use PIA and it did not on mine.

Regardless, neither of these would cause the screen issues you are seeing, especially since your iMac has a discrete graphics card built-in.

I tend to agree, if you have AppleCare please contact them ASAP. If you did not buy AppleCare well you may be about to find out why you should have.