Video quality is poor when played back on timeline

Hi.
How are you?

I uploaded a .mkv video ( webcam capture on ubuntu mate laptop). If i double click on the video clip in the time line it plays good. If i press the play button on the top right hand screen after selecting the clip on the time line is plays good. If i select the time line i.e. without clicking on the .mkv video it plays bad. The colour of the video is poor. If i export the video it records with the bad colour quality. I don't understand why the video is good when i double click on it but bad when I select the time line anywhere away from the .mkv clip

Is there a setting at i need to set in preferences to make this problem go away?

Thanks for your help with this.

Regards Sean.