Chlorophyll is a green pigment that gives most plants their color. The reason leaves appear green is because it absorbs other colors of light that is incident upon it but reflects only green.

The colour of an object as perceived by the eye is determined by

1. The incident light falling on the object.

1. The reflecting property of the object and

3. How sensitive the observer is.

In the case of plants, the incident light is sunlight which contains all wavelengths in the visible spectrum in nearly equal proportions. So, a full visible spectrum falls upon the chlorophyll-containing plant and helps it absorb specific wavelengths of light which is used to form the chemical bonds of sugar molecules by a process known as photosynthesis.

In plants, the particular wavelengths that re needed to form these chemical bonds are in the blue and red ends of the visible spectrum, so these wavelengths are absorbed (or subtraced) from the visible spectrum leaving only the green middle wavelengths to be reflected off to the observer. That is why plants are green.

However, when plants lose chlorophyll pigments in them, the leaves turn yellow and eventually brown. Without chlorophyll, leaves of plants won’t be green, and it would be difficult for the plant to trap solar energy  for photosynthesis, leading to death of the plant.