The Nature of Color
Light comes in the form of particles that come in waves. Light seen through a prism gives us a spectrum of different wave lengths, each producing a different color. We can see this prism in a rainbow. We see the light that is reflected, not light that is absorbed.
We associate the color green with chlorophyll, the substance in leaves that is responsible for photosynthesis, the process by which plants use energy from the sun to convert carbon dioxide in the air and hydrogen from water into sugars necessary for growth.
Pigments in Plants
Plants have three pigments that reflect basic bands in the color spectrum. These pigments in combination give us familiar plant colors. Chlorophylls absorb red and blue light. They reflect green light that they don't need for photosynthesis, so we see green leaves. Carotinoids absorb blue-green and blue light. They reflect yellow or yellow orange light, giving us yellow leaves in the fall and vegetables such as carrots. Anthocyanins absorb blue, blue-green and green light. They reflect red and purple light, hence red cherries and purple grapes.
Colors of Plants
Leaves are ordinarily green because chlorophyll reflects green light. Other parts of plants reflect different colors of light, hence flowers and fruits that are yellow, blue, orange, red and purple.
Use of Different Light
The longer the length of a light wave (the distance between the peaks and troughs), the more red is the color. Plants ordinarily use more blue light in their growing period in the spring and summer, and more red light when they are producing blossoms or fruits. As deciduous trees prepare to go dormant in the autumn, the dying leaves lose their chlorophyll and they turn yellow, orange, red or purple before they drop.
Purple leaves contain more anthocyanin than chlorophyll. The anthocyanin masks the chlorophyll so we see more purple. The chlorophyll is still there conducting photosynthesis. Purple leaves growing in the shade produce more chlorophyll. If examined closely, they are a muddy purple in color or even may look green. Likewise, apples often appear more red on the side that faces the sun and green on the shady side.