Plants on Earth are green because they contain chlorophyll. The chlorophyll is green because it absorbs mainly blue and red light in order to produce food for the plant via photosynthesis, while reflecting the green light frequency.
Scientists at NASA point out that if the stars for other planets were in a different state than our sun and if the light frequency that reached the planets' surface was different, then the plants would have also evolved a different type of photosynthetic pigment (other than chlorophyll). This pigment would be dedicated towards the different light frequencies received by the planet. This would cause plants to appear a different color from green, such as red and yellow.
According to recent studies, no photosynthetic plants would be blue-colored. This is because blue light provides some of the highest photosynthetic yields in the light spectrum. It is important for blue light to be absorbed rather than reflected. This is based on the physical quality of different frequencies of light produced by known types of stars.
One terrestrial example of energy conversion based on something other than ordinary light involves radiotrophic fungi that convert high energy gamma rays into useful energy using the melanin. (In most organisms melanin is used to protect the organism against ultraviolet and solar radiation.) Even still, ordinarily fungi derive their energy from decomposing other biomass, rather than by converting radiation into energy for itself.
It could even be possible for photosynthesis to occur using infrared light. In such an environment, plants may actually appear black.
It is fascinating to image the variations of life that are possible, even if life is based on the same fundamentals as our own.
Reference: Wikipedia article; Wikinews article; NASA - NASA Predicts Non-Green Plants on Other Planets; Dadachova, E; Bryan RA, Huang X, Moadel T, Schweitzer AD, et al. (2007). "Ionizing Radiation Changes the Electronic Properties of Melanin and Enhances the Growth of Melanized Fungi". PLoS ONE 2 (5): e457. doi:10.1371/journal.pone.0000457; Candace Lombardi "NASA: Plants on other planets not green". CNET, April 11, 2007; Julie Steenhuysen "New hue: Plants on other planets may be yellow, red". Reuters, April 11, 2007; Ker Than "Colorful Worlds: Plants on Other Planets Might Not Be Green". Space.com, April 11, 2007; “The Color of Plants on Other Worlds” by Nancy K. Kiang, Scientific American April 2008