In general, photosynthesis happens mostly via a mechanism of a green chemical known as Chlorophyll. However, that's not necessarily the only option for a photosynthesis chemical. Even here on earth, alternative photosynthesis could be based on Retinal or Melanin instead of Chlorophyll.
Chlorophyll is green because it absorbs more of the red and blue wavelengths of light as energy, and reflects more of the green wavelength. Older plant forms, such as lichen and moss absorb more of the blue spectrum. Certain bacteria deep in the ocean are more purple to our eye, and absorb/convert mostly infrared wavelengths. Snow Algae also absorbs infrared, and can result in red or watermelon-colored snow. This is in stark contrast to plants, which reflect infrared energy in a phenomena called the Red Edge. (The Red Edge also explains why vegetation is so bright in infrared photography - IR bounces off of it in disproportion to the actual temperature of the plant.)
If our atmosphere were just a little different, or our Sun burning a slightly different chemical mixture and generating light of a noticeably different color, our plants would be likely to have evolved to different shades than is the current average. In fact, it has been proposed that exactly this scenario happened in days gone by when our Sun was less bright and our atmosphere had a different composition. According to the Purple Earth Hypothesis, early life on earth used Retinal (aka Vitamin A) instead of chlorophyll, and the simple algae were purple instead of green. See also Halobacteria.
Another purple option is bacteriochlorophyll (similar chemically to chlorophyll, but absorbing somewhat different wavelengths of light). Combined with carotenoids, this is what makes 'purple bacteria' purple. Many purple bacteria use hydrogen sulfide rather than water (producing sulfur rather than oxygen) in their photosynthesis process.
Recently, Fungi has been discovered that uses melanin (a black pigment that humans, and many other species, use to protect against ultraviolet solar radiation) to conduct a photosynthesis-like reaction at Chernobyl to turn radioactive gamma ray emissions into food. See also Chernobyl Fungus Feeds On Radiation.
Of course, extraterrestrial life, using alien amino acids might develop other, more exotic compounds than just Chlorophyll, Retinal, and Melanin. Virtually any wavelength / pigmentation might come about given the breath of imaginary Alien Biochemistry to work from. However, science tells us that certain colors are more likely…
- Late stage Class M Stars, such as Red Dwarf Stars and Red Giant Stars, emit most of their light in the infrared spectrum, and radiate dimly in general. Therefore, plants evolving under such a star may develop a black pigment that absorbs as much light as possible, or even a pale pigment that does the majority of it's absorption in the infrared range alone.
- Plants on a planet orbiting an Class F Star would find it most efficient to produce a predominantly orange photosynthesizer.
- The blue wavelengths of light are more energetic and would make more efficient photosynthesis, so it unlikely that blue photosynthetic pigments would evolve or dominate (since a blue pigment would reflect the blue light instead of absorbing it). However, it's not inconceivable that a planet orbiting very close to a blue star, or being bombarded by radiation from a somewhat more distant Class O Star might develop blue pigmentation to deflect the strongest and harshest parts of the light. So while blue is the least-expected color for an alien forest, it can't be completely ruled out.
Game and Story Use
- Now you know what colors of plants are likely to show up on given worlds - and that it's directly related to (but not entirely dictated by) the color of their suns. This should make realistic world-building easier, and your science fiction settings more colorful.
- A life-bearting Goldilocks Planet orbiting a solar-twin is likely to have the same range of coloration found on earth, unless there's some atmospheric chemical or alien amino acids issue to explain otherwise.