@saomoon I had this problem, even though now not anymore. I don't know if you still have it or if this is going to work but I think it is the sidebar colour which shows on every video in full screen. I don't think it is a bug or a glitch, if you want to remove it I think you can hide the sidebar for the time you are watching the video and show it back when you are finished. To hide it by using a keyboard shortcut, you should go to settings and search "shortcuts". Then find "Enable advanced keyboard shortcuts" and turn it on. Then, under, click on "Configure shortcuts" and find "Show sidebar". Here you can configure what keyboard shortcut you prefer!