Tennis balls are made of a pressurized rubber core covered with a cloth material. As per the ITF(International Tennis Federation) manufacturing process this cloth can be of two types - Needle cloth and Melton cloth. Melton cloth has high wool content and Needle cloth can have a greater content of synthetic fibres. Tennis balls are usually yellow in color and that is the color sanctioned by the ITF for tennis tournaments. The ITF introduced yellow tennis balls into the rules of tennis in 1972 based on research that had shown these balls to be more visible to television viewers.
According to the ITF, the tennis ball must lie within the specifications specified in the Rules of Tennis, while providing consistent playing properties and being sufficiently durable to withstand repeated high speed impacts with the surface and racket.
As per the ITF, historically balls were either white or black in color, depending on the court's background color. The ITF introduced yellow tennis balls into the rules of tennis in 1972 based on research that had shown these balls to be more visible to television viewers. However Wimbledon continued to use the traditional white ball, but eventually started using yellow balls in 1986.
So yes, the official color of tennis balls is yellow(and not green).
The ITF has also introduced tennis balls which play 'slower' for beginners and recreational players who are learning to play tennis and they are red, orange, and green in color.
As per the ITF there is a standard (Type 2) ball that should normally be used for playing tennis but for different locations and court types additional types are available. For slower court surfaces there is a slightly harder, fast-speed ball (Type 1) and for faster courts there is a slow-speed ball (Type 3). Also, there is a ball specifically designed to be used in high altitude. High altitude balls were introduced into the ITF rules in 1989, and Type 1 and Type 3 balls in 2002.
To make it easier for beginner and recreational kids and adult players to learn tennis and make it more enjoyable, tennis balls have been developed that are designed to play 'slower' with the idea that it will enable longer rallies. The ITF has recognized three such groups of balls - Stage 3 (red), Stage 2 (orange) and Stage 1 (green). There are two sub categories of Stage 3 (red) ball - Standard construction and Cut or moulded foam. The idea behind this is for players to progress from using Stage 3 to Stage 1 and then start using the standard Type 2 ball. These balls are generally softer and lighter than standard balls and easier to control. Comparatively Stage 3 balls are lighter and bigger and Stage 1 balls tend to be heavier and smaller.