Leaves on deciduous trees change colors in the autumn months because of a breakdown of a pigment called chlorophyll. However, to understand the science behind this change, you should understand a few more things about the biology of leaves and what, exactly, contributes to the process of leaf change.
Photosynthesis is a process by which plants, including trees, use energy from the sun to convert water and carbon dioxide into carbohydrates, such as sugar and starch, which act as food for the plant. The process occurs in the leaf. More specifically, it occurs in the leaf's cells.
A leaf's color comes from several pigments that exist in the leaf's cells (the same cells that support photosynthesis). The dominant pigment is chlorophyll, which gives the leaf its green color. Chlorophyll is the most important pigment for a leaf. It is, in fact, chlorophyll that absorbs energy from sunlight. Without chlorophyll, plants could not photosynthesize and they could not subsist.
A leaf may have other pigments, including carotenoid, which gives leaves a yellow, orange, or brown color, and anthocyanins, which gives leaves a red color. These pigments are not exclusive to leaves: in fact, carotenoid is the pigment that gives carrots their orange color and anthocyanins make cranberries red. Normally, the chlorophyll overpowers the other pigments. So while a leaf may contain a certain amount of other pigments, it may still appear to be a brilliant green color.
As the seasons progress, days get longer and shorter. At different times of the year, the Earth rotates, giving some areas more sunlight per day and some areas less sunlight per day. During the summer months, daylight is longer and leaves can produce chlorophyll regularly.
But as autumn nears and leaves receive less daylight, the leaves stop producing so much chlorophyll. Eventually they stop producing it altogether. As the existing chlorophyll breaks down, the leaf's green color diminishes and disappears, leaving the other pigments, including those for yellow, orange, and red, to dominate.
After the chlorophyll breaks down, leaves turn a variety of colors. For example, the leaves of an oak tree turn mostly brown during the autumn because of the dominance of that pigment in the leaf. Others create brilliant purple displays that occur when any chlorophyll residue mixes with the other pigments, much like if you were to mix together remaining paint on a palate.
Autumn weather conditions, too, play a role in the progress of leaf color change. If the day is warm and sunny but nighttime temperatures dip below 45 degrees Fahrenheit, for example, the red coloration of leaves will be more extreme. This happens because sugars, which are made through photosynthesis during the day, leave the leaf when the nights are cool; any trapped sugars transform into anthocyanin, the pigment that gives leaves a red color (although it may turn the leaves violet or blue, also).
A tree's genetics, also, have an effect on its autumn colors. This is why leaves on a single tree may be a variety of colors. On a single tree, leaves that are exposed to more direct sunlight may turn red while leaves that receive more shade than sunlight may be yellow. Some trees have leaves that turn brown directly, never showing any other colors.
And a tree's leaf colors may differ from year to year. For example, during one summer, the conditions may be warm and rainy, which is likely to yield red leaves. The next summer the weather may be cool and dry, which will yield yellow leaves.
Most trees, however, have typical leaf colors that range in depth of color and not necessarily in range of color. Red maples, for example, may alter their leaf colors from dark red to light red, but they'll likely always be a shade of red, due to the pigments in the leaves.
Not all regions of the world experience autumnal color changes. While regions such as the eastern region of the United States have large areas covered with broad-leafed trees and weather conditions conducive to leaf color change, other regions, such as the tropical rainforest, have broad-leafed evergreen trees that shed their leaves very gradually. And in areas where most of the trees are evergreens, the leaves or needles will remain green or green-ish throughout the year, never changing greatly in color.