Q: iMac's screen turned green and now won't boot

Follow these steps:

- Shut down your Mac

- Wait until your computer turns off and after that press the Power button

- Right after you hear the startup tone, press and hold the Shift key

- Release the Shift key when you see a grey Apple sign and the progress bar below this sign

- Once you see Desktop, start a Disk Utility scan to detect and repair file system errors (don't forget to choose your main hard drive)

- Click on Verify Disc and then, if asked to fix problems, on Repair Disk

- After this, click on Verify Disk Permissions and then on Repair Disk Permissions

- After the process is finished, shut down your Mac and turn it back on after about 30 seconds

Apart from that, take a look at this Apple article and follow the instructions on Resetting NVRAM shown there: How to Reset NVRAM on your Mac - Apple Support

If this does not help, follow the steps suggested by Apple in this article:

https://support.apple.com/en-us/HT204267

Hope this helps!