The purpose of photosynthesis is to convert the energy in photons (the infinitesimally small packets of energy that make up light) into the chemical bonds of sugar molecules.

Plants(and animals that eat plants) can then store the energy and get it back out when they need it by breaking those chemical bonds. The tricky part of photosynthesis is that it takes a very precise amount of energy to form a particular chemical bond. Furthermore, the photons from different colors of light contain different amounts of energy.

You probably know the colors of the spectrum (Red, Orange, Yellow, Green, Blue, Indigo, Violet); well, those colors are in ascending order of energy -- a photon of blue light has more energy than a photon of red light (this is true because of Planck's Law, which a physicist could explain better than I).

A particular pigment molecule (like chlorophyll) is specialized for absorbing a particular color of light and converting the light energy into the appropriate amount of chemical energy for making a chemical bond (actually there are many other molecules that help the pigment perform this energy conversion, but only the pigment itself can absorb the light). Chlorophyll just absorbs blue and red light; it hardly absorbs any green light at all, so the green gets reflected back to our eyes, which is why leaves appear green.

Other pigments that plants have in their leaves absorb light of different colors, so they reflect red, orange, yellow, or blue light and appear to be those colors to our eyes. Because the white light coming from the sun is actually made up of photons of all the different colors, it is very advantageous to the plant to have many pigments that can absorb such a wide range of the available colors of light.

Chlorophyll is a green pigment that gives most plants their color. The reason that it is green is because it absorbs other colors of light such as red and blue, so in a way the green light is reflected out since the pigment does not absorb it. Plants contain other pigments that reflect different colors, but these are often masked by Chlorophyll.

An example of another common pigment is the carotenoids. During autumn, when the days begin to shorten, the amount of chlorophyll in the leaves of trees is dramatically reduced. This is when the colors of the carotenoids become more prevalent and is what you see when the leaves appear to change color.

Different photosynthetic organisms use different combinations of pigments, which have different colors because they absorb and reflect different frequencies of light.

Plants and green algae (plants are really advanced green algae) contain chlorophyll a (which is teal-green), chlorophyll b (which is yellow-green), and beta-carotene (which is yellow), thus giving them a green color. Brown algae and their unicellular relatives (e.g. diatoms) have chlorophyll a,chlorophyll c, and lipid pigments called fucoxanthins, which together give them a golden-brown color.

Red algae possess chlorophyll a and lipid-based pigments called phycobilins, which give them the brilliant red(or deep blue) color. These different combinations of pigments are more or less efficient at collecting light at certain frequencies and at certain levels of light intensity (too much will damage the pigment). Thus, they parcel out the Sun's energy to make the most use of it and to not compete with other photosynthetic organisms.

I'm a little confused about your question. Chlorophyll pigment is always green. Plant leaves and stems aren't always green because they have many pigments other than chlorophyll.

Pigments are molecules that absorb specific colors of light and reflect other colors, depending on their chemical structure. The reflected colors are what give pigments their color. Chlorophyll pigments are green because they reflect green light. There are different types of chlorophyll (chlorophyll-a, chlorophyll-b, chlorophyll-c1, chlorophyll-c2, chlorophyll-d, divinyl chlorophyll-a). These different types of chlorophyll are the same basic molecule with very slight differences in their chemical structures.

Because the different chlorophylls have the same basic structure, they all reflect green light and so appear green, but their small structural differences cause them to be different shades of green (yellow-green, lime green, forest green, blue-green, etc.).

To see the structure of chlorophyll, go to: Chlorophyll

Land plants (and plants in the ocean, called algae) have a lot of chlorophyll-a pigment because it is essential to photosynthesis, but they also have other pigments, called accessory pigments, that help them absorb light. These accessory pigments can be other chlorophylls or they can be completely different pigments with completely different colors, including yellow (xanthophylls), orange (carotenes), red and purple (phycobilins) or brown and gray (phaeophytin).

Most land plants are green (their stems and leaves, anyway) because the accessory pigments are chlorophylls, including chlorophyll-b and chlorophyll-c. When green leaves "turn colors" in the fall, or turn yellow due to nutrient limitation or disease, the chlorophylls are breaking down and being re-absorbed, allowing the other pigments to show through. If you've even been scuba diving or snorkeling, you may have noticed that only some algae in the ocean are green and that most are brown, red, purple, yellow or even iridescent blue. The study of land plants is called botany and the study of algae is called phycology. I myself am a phycologist because I study marine algae.

To read more about photosynthetic pigment, including chlorophyll, go to: Photosynthetic pigment