Plant Colour

Chlorophyll

Google it.

Not all do.

See chlorophyll

Very true...

Sand Cherry tree

Bloodgood Japanese Maple tree

Wandering Jew plant

(All images off the web, public domain afaik.)

Forgot to label my picture as dusty miller _{<which I plant because it's one of the few things deer won't eat>}

Plants, like everything else, absorb certain colors of the spectrum and reflect others. When you see a green plant, it is the color of the spectrum that is being reflected back to your eyes.

That applies to any colour, not just green. It doesn't explain why (most) plants are green.

"...green plants use a complex chemical known as chlorophyll during photosynthesis.^[17] Chlorophyll does not absorb green light because it first arose in organisms living in oceans where purple halobacteria were already exploiting photosynthesis. Their purple color arose because they extracted energy in the green portion of the spectrum using bacteriorhodopsin. The new organisms that then later came to dominate the extraction of light were selected to exploit those portions of the spectrum not used by the halobacteria."
http://en.wikipedia.org/wiki/Green

Because God was a greenie, and made them that way.

This is an interesting point and is not answered in web too. There is an interesting article here but that too doesn't explain.

The most interesting part is the bathtub shaped absorption spectra. The highest energy (read) and the lowest energy (Blue). And it makes one think.

I can make two hypothesis -

a) The day time the plants tried to avoid the highest energy density middle spectrum to save themselves from being burnt and are happy with the high energy red in moderate quantity. As night snacks they must have the blue absorption pigments.

b) The excitation levels of the electrons might match the two wavelengths and then they would conveniently skip the others.

I couldn't fine anything in layman's terms on these two aspects and couldn't even find the excitation levels of the proties (some of the papers might have but they want $s which I am reluctant to spend in a subject that is not my field)

There may be other miriads of possibilities too but what most of the papers and articles say I don't accept.

a) Just it is (as some members mentioned above)

b) It is the colour of the pigment. Accepted but then why?

b) It is from the ancestral when plants were below water. but then they would have changed and mutated as they started getting unmerged (like the barks becoming brown). Mr Darwin (In whose honour the award was constituted) would have forced them to become extinct and removed them from the gene pool.

But unfortunately it might not be a topic for this forum may be physics forum (_{but there too similar to above answers were given})