why plant cell is green?

that's right,Chlorophyll is something which makes plant cell green,but not all plant.in fact ,some plants don't have chlorophyll!so they may are red or white in couour.in fact,i think that you may want to know more about it,right?Chlorophyll is a green pigment found in most plants, algae, and cyanobacteria. Its name is derived from Greek: χλωρός (chloros "green") and φύλλον (phyllon "leaf"). Chlorophyll absorbs light most strongly in the blue and red but poorly in the green portions of the electromagnetic spectrum, hence the green colour of chlorophyll-containing tissues like plant leaves.It's really important for plants.

Chlorophyll is a chlorin pigment, which is structurally similar to and produced through the same metabolic pathway as other porphyrin pigments such as heme. At the center of the chlorin ring is a magnesium ion. The chlorin ring can have several different side chains, usually including a long phytol chain. There are a few different forms that occur naturally, but the most widely distributed form in terrestrial plants is chlorophyll a. The general structure of chlorophyll a was elucidated by Hans Fischer in 1940, and by 1960, when most of the stereochemistry of chlorophyll a was known, Robert Burns Woodward published a total synthesis of the molecule as then known In 1967, the last remaining stereochemical elucidation was completed by Ian Fleming,and in 1990 Woodward and co-authors published an updated synthesis

Not all plant cells are green.
Usually, only those parts that need to perform photosynthesis that are green e.g. leaves. Because they contains chlorophyll, usually green in color, which is required for photosynthesis.
Some plant cells are not green, e.g. stem cell and root cell.
Also, mushroom are not green either.