One of the most defining features of autumn is the leaves transitioning from a green colour, which we are more accustomed to, to a deep shade of yellow, orange or red. Despite this colour change seeming visually simple, the chemistry behind it is far from that.

Molecules being able to show colour is because of conjugation: systems of alternating single and double bonds. A large amount of conjugation within a molecule means that it can absorb and reflect wavelengths of visible light and present colour.

Chlorophyll is commonly known as an essential component in photosynthesis, however, it is also responsible for the green colouring in leaves before they change colour in autumn. Warm temperatures and sunlight are needed for chlorophyll production, and so as summer ends, the rate of chlorophyll decomposition is greater than the rate of its production so the green colour fades as the colder and darker climate in autumn begins.

Carotenoids and flavonoids are both present alongside chlorophyll in the summer, but the higher levels of chlorophyll masks their colours. In the autumn, when chlorophyll levels drop, the colour begins to change, both families contribute yellows while carotenoids also contribute oranges and reds. Aside from changing the colours of leaves, carotenoids are also responsible for the colour of tomatoes (lycopene), carrots (beta-carotene) and egg yolks (lutein).

Anthocyanins are a member of the flavonoids family and are responsible for the deep purple colour of leaves. They do not exist in leaves all year round and their synthesis begins when the concentration of sugars in the leaves increase. Their precise role is not certain but it is suspected that they help to protect the leaves from light damage and therefore increase the amount of time before they fall from the tree.