How does correct gamma color on external screen? Linux laptop
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I have got a laptop and i would like to correct gamma color on my **external** screen.

My system: *Laptop, VGA port, Linux, Ubuntu.*

In option System->Preferences->Monitor i turn off my laptop's screen.

But bash-command  `xgamma -gamma 5` does not any effect. BTW on laptop is (if it will turn on) too.

**How can i get it?**