It seems obvious to me that a red line indicates a bad move and a green line a good move. Graphically this shows in the fact that steep lines are red and horizontal lines are green. This might be slightly confusing because the graph shows the moves of both, black and white.
Because an engine always assumes best play when evaluating a position you can never improve your position by playing a move, best case scenario is you play the best move and the eval stays the same -> horizontal line. A mistake on the other hand changes the evaluation -> the line goes up if black moved, or down if white moved.
There is probably a cutoff for what constitutes a bad move, something like a 1.0 pawns evaluation drop seems likely.