How do I fix a colored vertical line on my iMac screen?

First, let’s see if this issue happens in Safe Mode:

1. Shut down your Mac.
2. Press and hold the power button on your Mac. As you continue to hold the power button, your Mac turns on and loads startup options. When you see Options, release the power button.
3. Select your startup disk. It's named Macintosh HD, unless you changed its name.
4. Press and hold the Shift key, then click the “Continue in Safe Mode” button below your startup disk.
5. Log in to your Mac. You might be asked to log in again.

If the issue doesn't continue in safe mode:

Leave safe mode by restarting your Mac normally, then try to reproduce the issue.

If the issue doesn't return, it might have been resolved by the other things that safe mode did at startup, such as checking your disk and clearing caches.

The last resort:

You are facing some sort of internal failure.

Get expert service and support at the Genius Bar.

Service Checklist:

• Mac service pricing.
• Check AppleCare Coverage.

Click ▶︎ Start a Service Request. ⚛️

Need more help?

• Chat now or call 1-800-APLCARE.
• International Support.

Click ▶︎ Check Repair Status. 🔧

That typically indicates a hardware failure. Make an appointment at the Genius Bar.

Thanks for the feedback, faisallizan!

Be sure to follow up with a service appointment as I had originally suggested and let them know the steps you have taken.

Good luck! 👍🏼

Try the suggestions that @Smilin-Brian suggested.

If the 'colorful vertical line' continues, it's probably a hardware issue with the display controller or the display panel.