One common theory giving an answer to the question "why plants evolved to be green" is as follows : there is a peak in solar radiation intensity in the visible spectrum around yellow-green, and plants reflect that portion of the spectrum in order not to overheat, or overcharge.
The green is clorophile, which is also the molecule producing complex organic compounds from CO2 and oxygene. There are two types of clorophile, each reflecting a certain portion of the spectrum; theory says they started getting closer to green when plants synthetising non-green clorophile were burned out by the solar radiation, because their sensible proto-clorophile molecules broke down into smaller compounds. Thus, by applying evolution theory when stable clorophile appearred, it was green
So, in evolution terms, it's all a problem of energy balance. On a planet orbiting a cool red star, plants or plantoids would barely get enough energy, so they would evolve into black (absorbing the whole spectrum). On a planet orbiting in the inner ecosphere of a hot red star, plants would reflect some portions of the spectrum so that they wouldn't burn down - they would be red, or orange, or magenta. Around white or white-blue stars, you may even get to find bright-grey plants, and so on.
I guess a competent biochemist would shed more light on the subject.