Green stars do not naturally exist in our universe. Stars emit light across a broad range of wavelengths, and the color we perceive depends on their temperature. The majority of stars, including our Sun, emit light that appears white or slightly yellowish. Cooler stars, such as red dwarfs, appear reddish in color, while hotter stars, like blue giants, have a bluish hue. However, the color perception of stars can also be influenced by the atmosphere and other factors.
As for green planets, the color of a planet is primarily determined by its composition and atmosphere. In our solar system, most planets have distinct colors. For example, Earth appears blue due to the prevalence of water, while Mars has a reddish appearance due to iron oxide on its surface. Jupiter and Saturn have bands of different colors caused by their atmospheric composition. While green-tinted planets are not common, it's theoretically possible for a planet with a unique combination of atmospheric gases and reflective surfaces to appear green. However, we have not discovered any confirmed green planets.