Why are trees green?

I assume you mean why are leaves green, right now most trees are brown, well, the deciduous trees are brown.  Evergreens stay green.  Except the blue spruce, which is more a bluish-green.  Palo-Verdes also stay green, which just shows that there are always exceptions to rules.

Well, back to the question.  Leaves are green because chloroplasts are green (leaves are full of chloroplasts).  Chloroplasts are green because chlorophyll is green.  So, the real question is what theory do you believe in, evolution, or intelligent design, or something else?

The biblical account of the creation tells us the order things were created.  First light and dark, the sun is yellow, so yellow was out for leaves as was black.  Next the earth was created removing brown for dirt, and gray due to rocks.  The combination of earth and sun makes sunsets, this removes red and orange from possibilities.  Also with the earth is the sky, which is blue.  So, in the end the only choices were green and purple.  We are left to assume that green was chosen for being more "gender neutral."  I am just glad that we humans did not get stuck with purple, that could have been ugly. 

Evolution would say that it is just due to random chance, some protein worked and nature stuck with it, making steady improvements over millions of years.  Bah.  Wheres the science? The intelligent design used the process of elimination to reach the conclusion.  I think it was the wise Sherlock Holmes who said if all the probable causes have been eliminated then the improbable must be true. 

So there you have it.  Trees are green because it was available and gender neutral.