Thank you Everest for your suggestions about sequence settings.
You see, FCP changes the settings by it self and finds itself the most suitable codec.
So either manually or automatic the problem remains unsolved.
in my case, it was a QuickTime Uncompressed movie in HD format. So if FCP has that problem, it means that it is impossible to Color Correct any footage.
Yes, every setting is in full quality, and without adjusting anything,
when i switch between color and qt , there is always a slight difference.
color;s preview has inferior quality…