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Check Display setting in your System Preferences pane?
If the Display Profile was set to any other than your mac setting, try to set it back to your mac’s default.

For instance:
System Preferences > Display > Color > Display Profile > iMac.

It worked for me. Hope this helps.

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[Bounty Umbara]

I had the same problem.
Mine is Yosemite on iMac late 2013.

I always turn on my external monitor first before I open FCP 7, but a few days ago I was in hurry to see some cuts in an FCP project without turning on my external monitor. There I saw my whole sequence looks much darker in Preview window.

There was no setting changed in FCP 7, all settings were set as how it was before. So I didn’t touch any setting there.

However I found one thing changed in (OS X) System Preferences pane, the Display Profile, It was set to HD 709-A, not iMac as how it was set before.

So I changed it back to iMac.
It solved my problem.

System Preferences > Display > Color > Display Profile > iMac

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