Why is the grass green?

Chlorophyll is a green coloured pigment that is prevalent in grass.

Because if it were pink, it wouldn’t match the trees.

To answer this you must understand that an objects color is the surface of that object reflecting different wavelengths of light to your eyes. If something is green it absorbs all wavelengths of light except green.

Plants have evolved to utilize all wavelengths of light except green. Had plants evolved to use all wavelengths of light except purple, all plants would be purple.

Because it is envious of the sky.

Grass is green because it contains chlorophyll, which is green. The chlorophyll makes nutrients (sugar) for the plant from the light of the sun (photosynthesis).

For the same reason that the sky is blue. Google or go to Wickipedia, please.

Why do fools fall in love?

The core of clorophyll, the marvel behind the process of photosynthesis, consists primarily of Hydrogen, Carbon, Oxygen, and Nitrogen atoms arranged in a ring pattern. At the exact center of this complex molecule rests a single atom of magnesium- a dull-gray metal that gives much of the world its leafy-green color. Substitute that single magnesium atom for one of iron and presto!- a clorophyll molecule magically transforms to resemble closely the core of hemoglobin. Iron ore, another dull-gray metal, plays an important role in coloring the innards of all complex animals red. Green for plants; red for animals. So beautiful, so simple.

Some sea creatures—crabs, for example—use the organic molecule hemocyanin to oxygenate their blood. Remarkably similar to human hemoglobin, hemocyanin uses copper at its core instead of iron and that copper colors the blood green. Perhaps the reason that monsters in film and computer games seem always to have green blood.