Upper and Lower Band is filled with blue color to indicate of price movement.
Line chart is represent price movement.
Decision to buy/sell is as following :-
1 - Potential Buy - Price Line turned Green crossed above upper Band. The important things is buy at squeeze for low .
2 - Potential Sell - Price Line turned Red crossed below lower Band.
Note : Price breakout from narrow band normally result in rally.
The longer duration of narrow band will result in stronger rally after breakout.