Suggested by Oliver Stephens in his article for TennisPro Magazine. Players should be able to rally
As soon as a ball is struck by the opponent, the other player calls out either Red, Yellow or Green depending on how he/she evaluates the incoming ball. Red is for a deep difficult ball, meaning they 'stop' whatever offensive intentions they may have had and just hit the ball to stay in the point. Yellow is for a mid court or transitional, neutral ball. Green is a ball they feel they can or should attack offensively, probably a softly hit ball, or short, or both.
In addition to recognizing what ball is coming their way, players should respond accordingly and hit the correct type of ball back.