Chlorophyll is a green pigment in the leaves. When trees stop producing chlorophyll in the fall, the green pigment stops filling the leaves, so the natural colour of the leaf is revealed. Some common leaf colours are orange, yellow and brown.

Red fall leaves are caused by production of anthocyanin. Anthocyanin is a reddish-purplish pigment that helps trees protect themselves from insect pests over the winter.

The declining amount of sunlight each day in the fall is the main trigger of colour change. In addition, warm fall temperatures can delay the end of chlorophyll production and leaf colour change. Here’s an interesting fact: Trees start their colour change in late June after the summer solstice, when the minutes of sunlight start to decline. It’s a slow process at the start so the colour change normally does not become noticeable until September or October.