Leaves get their green color from chlorophyll, contained in cells within the leaves. Chlorophyll is necessary for photosynthesis, it captures sunlight to produce plant sugars. As days grow shorter and temperatures cool in the fall, trees stop producing chlorophyll, some earlier than others.
Without chlorophyll to capture sunlight, other pigments contained in the cells of leaves become more visible. These other pigments are called accessory pigments, they include carotenoids and anthocyanins.
Carotenoids absorb blue and violet light and reflect yellow and green wavelengths, which is why they appear yellow to the human eye.
Anthocyanins absorb blue and green light and reflect red wavelengths, which is why they appear red to the human eye. These accessory pigments are present in leaves all year long and some accumulate higher than others in the fall.
Why Are The Trees More Colorful Some Years?
The amount of sunlight and temperature conditions determine if trees will turn more colorful in the fall. Trees that grow at higher elevations or towards the poles may turn red and yellow earlier than those growing closer to the equator, which is why we see some trees with brilliant colors while others nearby are still green.
Also, years with abundant rainfall will produce more colorful fall leaves, while dryer, or even drought conditions will cause trees to skip their brilliant color phase in the fall.