Why are plants green? Why reflect that light rather than use it?

It's an interesting question. Not all plants are green; there are plants in the sea that are brown and red. They photosynthesise in just the same way but they use pigments that trap different wavelengths of light to get that energy. This shows that you can obviously do it with other wavelengths and it's not something specifically about green. It may just be something else about the early green sea plants that were more successful than the early red and brown sea plants that made them more successful and able to get onto land. So although they look the dominant form, the fact that they are green may not make them any better at all; it could be for some other reason.