I found out not long ago why the trees are green and wrote a poem about it. Trees are green because the leaves in manufacturing wood have no use for that particular colour of the spectrum, therefore it's excluded, left behind, and all over the place in summer.

Green

Tree leaves which, till the growing season's done,
Change into wood the powers of the sun,
Take from that radiance only reds and blues.
Green is a colour that they cannot use,
And so their rustling myriads are seen
To wear all summer an extraneous green,
A green with no apparent role, unless
To be the symbol of a great largesse
Which has no end, though autumn may revoke
That shade from yellowed ash and rusted oak.