Trees and plants are green because of a green pigment called chlorophyll. This pigment absorbs red light the best, and converts the light into energy that it uses for metabolism. As you likely know, this pigment allows plants to use light as a form of energy, as a part of a process called photosynthesis.

Instead of eating food to build molecules, plants can take light from the sun and use the energy to convert carbon dioxide from the air into useful molecules! However, the pigment doesn't strongly absorb blue or green light, so plants can't use this energy for photosynthesis as effectively. Interestingly, we know this even from the colour of plants! White light contains all the colours, and plants appear green because they absorb the red light, leaving what appears to us as green light, to be seen by our eyes! If the light isn't being absorbed by the plant, it can't be used for photosynthesis!

Although I've specifically talked about chlorophyll, plants have developed several different pigments that absorb different colours, but chlorophyll is certainly the most common.