I have tried everything and it still cannot get resolved. Once it stopped happening, the computer now flickers (not flashing) in a manner how sometimes phones with low battery act. ( this was happening as if the computer was trying to work itself around for color matching) then suddenly, odd colors started PPEARING.GREEN TINGE.TURNED RED.AND THEN YELLOW.THEN BLACK AND WHITE.THEN FINALLY STABILISED LATER. I was using the iMovie.got flickers and then my computer froze.

You may want to contact Apple or make a genius appointment. If you are still experiencing the problem, it is highly likely that you are having a hardware problem. Check to see if flickering occurs? if not, delete the new user. Then after, login to your computer as this new user. Create a user account on your Mac by going to System Preferences > Users & Groups.Try uninstalling Firefox to see if that fixes your flicking problem. It seems that, for some users, Firefox is causing this.After regularly starting your Mac, does flickering come back? if not, that means that safe mode fixed it. Does flickering still happens? If flickering does not happen, restart your Mac regularly (without pressing any keys).

Then you will see the login screen (now you may release the Shift key) and login your Mac.

Apple logo will appear (do not release the Shift key yet)

Start your Mac and immediately, press and hold the Shift key.

You can use safe mode to identify your issue.

On your Mac, go to System Preferences > Energy Saver > and uncheck the option for “Automatic graphics switching” (note that Mac Air models do not have this option, Mac Book Pro models have this option).

Start your Mac, and immediately press the Option, Command, P, and R keys together for several seconds (around 20 seconds).

Different Mac models require different steps to do this.

Reset the System Management Controller (SMC).

You can do so by opening the App Store on your Mac.

Restart your Mac by going Apple menu >Restart.

Please try each step until you resolve your problem: Here is what you can do if your Mac screen is flashing or flickering: Screen flickering issue, fixes Mac screen flickering can be caused by different issues. It further seems that some screen flickering issues are so severe that it makes it almost impossible to look at the screen. It seems that both retina and non-retina screens may experience this. Some users have started that the problem would occur randomly. Several MacBook users have reported that their screen displays are flickering.

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