y iMac keeps getting a purple screen and restarts

Unplug all external devices except of the power cord and test.

If it startup fine, then you have a bad cable or device that needs to be replaced.

If it still gets purple screen and restarts, then it sounds like a hardware issue that needs service.

To get Service:

Contact Apple Support for help finding a service provider.

https://getsupport.apple.com/?caller=arshw

Or contact your local Apple Service Provider and schedule an appointment.

https://getsupport.apple.com/repair-locations?locale=en_US

Contact Apple Support for help finding a service provider.

https://getsupport.apple.com/?caller=arshw

Or contact your local Apple Service Provider and schedule an appointment.

https://getsupport.apple.com/repair-locations?locale=en_US