Low compression tennis balls are the BEST way to teach young kids the great game of tennis! Because the balls are two-toned, bounce lower, and travel through the air much slowly, it allows kids to follow the ball much EASILY. This makes it so that kids can achieve success more often, making it more FUN, and sticking with the game longer. They are also great for adults who are learning the game.
The RED balls are 75% reduced compression than a traditional YELLOW tennis ball, and are recommended for beginners. The "36" is the size of the court you should be using the ball on(36 feet x 18 feet). There are two versions of the RED ball. The FOAM ball is generally for indoor use, and the FELT ball is for outdoor use. They both bounce the same, but the foam one will fly all over the place outdoors because it is bigger, therefore has higher wind resistance.
The ORANGE balls are 50% reduced, and are great for kids who have learned all the strokes and are VERY comfortable with the RED balls. i.e. They can hit topspin, serve, cross-court, etc. naturally around the court. The ORANGE ball should be used on a 60x21 sized court.