why is the color of a leaf green?

Its basic science. Leaf is green because of a pigment present in it called chlorophyll. Pigment is an organic substance (a chemical present in living things). It imparts colour to the leaf. It does so by reflection green light and absorbing all the other colours so you percieve the colour that is reflected. You might know that sunlight has different colours and you percieve colour according to the colour reflected by the substance. There are some leaves that are not green that again is the work of pigments but this time its not chlorophyll. It is some other pigment like Xanthophyll and Carotene. You might have noticed that leaves turn yellow and orange when they get old. This is again the magic of pigments as the leaf grows old its chlorophyll is depleted and gives way to other colouring pigments. Sometimes the leaves get yellow when they are exposed to excess sunlight this is called etiolation.

Every autumn we revel in the beauty of the fall colors. The mixture of red, purple, orange and yellow is the result of chemical processes that take place in the tree as the seasons change from summer to winter. During the spring and summer the leaves have served as factories where most of the foods necessary for the tree's growth are manufactured. This food-making process takes place in the leaf in numerous cells containing chlorophyll, which gives the leaf its green color. This extraordinary chemical absorbs from sunlight the energy that is used in transforming carbon dioxide and water to carbohydrates, such as sugars and starch. Image of trees changing colors in the fallAlong with the green pigment are yellow to orange pigments, carotenes and xanthophyll pigments which, for example, give the orange color to a carrot. Most of the year these colors are masked by great amounts of green coloring.

I was going to reply to this but then read the response from waytovj at the top and he/she put it so much better than I would have that I will leave it at that.

The plant contains chlorophyll which gives it it's green colour. Plants with leaves of other colours, still have the chlorophyll, but it's masked by other pigments. As to why plants have different colours, I could not say. You'll have to ask a botanist.

The thing which makes most plants look green is a pigment called chlorophyll, which helps them use light to make their own food. The reds, purples, oranges, and other colors which you see in plants are all caused by other pigments, each of which do different things.

The chemical that gives plant's their green colour is chlorophyll (the stuff they use to make sugars from sunlight). Chloropyll is green in colour, leaves are full of it to do their job of making food for the plant, so green leaves are just practicality. Plants with other colours in their leaves also have chloropyll to make their food, but have other substances in the leaves that give the coloration. Most plants that stay a different colour are usually a dark purple colour. This has a lot of green in it, but red etc too.