The NYSE imbalance alerts only happen near the end of the trading day.  These are based on the specialist's market on close orders.  If there are more people trying to buy a stock than are trying sell the stock, then we call that a "buy imbalance."  If there are more people trying to sell, then we call it a "sell imbalance."  The description of the alert includes the size of the imbalance in shares.

When there are more buyers than sellers, that often pushes a stock price higher.  When there are more sellers than buyers, the pressure goes in the other direction.  The imbalance data speaks directly to these facts.  That's why we use green to show buy imbalances, and red to show sell imbalances.

More options related to these alerts are listed below.