A person who's face has turned a red/purple because they are embarrassed, 'beetroot' is usually shouted when someone does something embarrassing and try's to pass it off.
(school kid drops food tray onto floor and his face turns red)
Guy 1: WWWAAAYYYY!!
Guy 2: WWWAAAYYY!
Guy 3: (aside to friend) his face is beetroot
Guy 2: haha ye
Guy 1: (shouted at kid) BEETROOT!!!
Guy 3: BEETROOT!!!!