Purple plants on a planet orbiting a green star?
Before I continue, yes, green stars don't really exist visually. Although stars like the Sun are green stars, they appear white (and are known as yellow dwarfs) because it doesn't only emit green light (which is the most intense set of wavelengths it emits) but also the colors around it. Through color addition this makes white. In this situation however let's suppose this star MAGICALLY is green.
However I want to focus on plants. Star type is a major factor in determining what the plant color is (it is not the only factor but let's skip all those exceptions). Under a red star, plants would appear very dark, while under a bright blue star they would rather be white colored. On Earth, plants are green since the Sun mostly emits green light, and thanks to chlorophyll, plants absorb red and blue light while reflecting the green light.
Since the star is green, it emits almost entirely green light, and I'd expect that only a tiny amount of blue and red wavelengths are emitted. I would expect the plants to absorb the green light and only reflecting the blue and red, and perhaps appearing magenta/violet.
Would magenta be the correct color of plants if they grew under a green sun? If not then what color would plants be under a green sun?
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To determine the color of the plants most likely to arise on a planet orbiting the star, we have to consider the wavelengths of peak absorption of different photosynthetic pigments. I would argue that different star types do indeed influence pigment choices.
Chlorophylls respond well to light around 435 nm and 650-680 nm; 500 nm - green light, approximately - falls in the middle. This helps the development and growth of green plants and green/red algae. If we wanted a different color for our plants, we'd need a different pigment. Purple is a sign of either bacteriochlorophyll a or bacteriochlorophyll b. The former has absorption peaks from 800-900 nm, while the latter has peaks at 435 and 740 nm. Spirilloxanthal, active in the 400-550 nm range, can also produce purple coloring in bacteria.
To have the majority of your plants be purple would require peak wavelengths for the star in the 800-900 nm range. This corresponds to a late K-type star or an early M-type star, which on the main sequence be red-orange in color, low-mass, and long lived. On the other hand, if you wanted magenta, you'd also need red pigments to be active. There are a long list of these, including the chlorophylls. Emission at 500 nm would indeed encourage the use of spirilloxanthal, depending on the exact wavelength. The problem is, it would also encourage the use of all the other pigments active in the range, including the chlorophylls. Some plants will be purple or magenta; most won't.
It's worth noting that Sun-like stars - as well as more massive stars - may grow redder as they evolve, ascending the red giant branch. Red giants aren't particularly conducive to life, but they certainly have wavelength ranges that would encourage bacteriochlorophyll production and use.
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