All leaves have some yellow pigment in them, and many have red tones as well. But these colors can't be seen in the spring and summer because they are overpowered by the green color of chlorophyll. Chlorophyll cells need sunlight to do their job. Once there is not enough light, leaves stop producing food and lose their green color. Then the other colors can be seen.
Leaves become yellow or brownish red.
Many of the bright fall color have not been present all along. They are made once the chlorophyll is gone. Now other chemicals have a chance to react with sunlight. Bright red and purple colors are the result. The sugar maple, for example, is one of the most colorful fall trees.