red screen on startup
Start up the screen is red. Also in time turns green and then back to red. Cursor moves but nothing shows up to select
iMac, iOS 12.1
Question marked as
That’s probably a hardware problem with the display, or with the graphics controller, or maybe a problem with the video cable if this is an external (analog) display monitor. You can try an NVRAM and an SMC reset, but those are unlikely to help. Also try a Safe Mode boot, hardware diagnostics. (The diagnostics aren’t always reliable, and might not find a fault.)
Reset NVRAM or PRAM on your Mac - Apple Support
How to reset the System Management Controller (SMC) on your Mac - Apple Support
Use safe mode to isolate issues with your Mac - Apple Support
How to use Apple Diagnostics on your Mac - Apple Support (June 2013 and later Mac)
How to use Apple Hardware Test on your Mac - Apple Support (Mac older than June 2013)