Chlorophyll and other pigments in green plants absorb blue and red light to power photosynthesis, and reflect away green light. The combination of colors used by plants is referred to as photosynthetically active radiation (PAR). We don't really know why the only pigments plants have don't use much green light, but it might be related to the presence in Earth's early oceans of microbes that used green light as an energy source and had it monopolized. The earliest green plants might have been better able to compete with ancient purple Archaea by exploiting the unused blue and red wavelengths. Some surviving Archaea still use green light today, but the photosynthetic pathways supported by the pigments in green plants are much more efficient.
We do have a good understanding of which wavelengths of light are used by plant pigments today. A standard high school or college lab exercise is to extract the pigments from plant leaves and separate them using chromatography or describe them using spectroscopy. The extraction is typically done with nasty solvents, and one lab manual calls for a mixture of petroleum ether, cyclohexane, ethyl acetate, acetone, and methanol. I'm sure there is a reason to use those solvents, but this afternoon I put a handful of spinach leaves in the blender with some cold water and made a lovely green slurry. I filtered it through a gold tea strainer, diluted it, and put a jar of the emerald liquid in front of Ebert's entrance slit. I also captured a spectrum of an identical jar of water.
I uploaded both images to Spectral Workbench and then downloaded the raw data and put it in Excel. I used the same data procedure that I used on the maple syrup results: for each wavelength value I used the intensity value for whichever color channel was highest, then I smoothed the series with a 10 value running mean. To represent how much light of each wavelength the green spinach liquid was absorbing, I subtracted the spinach transmission intensity values from the clear water transmission values.
The result looks a little bit like the standard absorption curve for plant pigments. It certainly depicts less absorption by green light, and more by both blue and red. I am not sure why the absorption at the red end of the spectrum is not as great as in the standard curve. Maybe there was not as much chlorophyll a as chlorophyll b. Or maybe the white balance of the digital image is to blame. Or maybe the artifacts associated with combining the three color channels are responsible. Or maybe I need more toxic solvents to extract the pigments.