Haha... that's actually a really good point. Right now, you've got me thinking, and it's raising more questions than it's answering.
With Earth's sun and current knowledge of photosynthesis, green plants don't make a ton of sense. Red or blue would seemingly take in more energy, since green is right near the middle of the sun's average yellow light output.
I've got to assume that the Andalite sun(s) put out light on similar wavelengths to Earth's sun, since Andalite vision does not seem to vary appreciably from human version in terms of frequency range (the vision of every animal on the planet is centered around the light patterns our sun outputs most strongly- what we see as 'visible light').
Basically, I think Earth plants really are just green because the chemical composition of chlorophyll makes it
a) the most efficient method on Earth of synthesizing energy from sunlight and
b) green
If plants are able to evolve to include a photosynthetic chemical aside from chlorophyll (and there are some species of 'photosynthetic' microorganism that are purplish in color), it makes total sense for them to be able to become the dominant species.
