Why do leaves change colour in Autumn?

The answer to this is that leaves look green because they contain the pigment chlorophyll which is what they need to do photosynthesis, the process of grabbing energy that's in sunlight and driving a chemical reaction with it in order to turn carbon dioxide and water into glucose – C6H12O6 – that's what photosynthesis is.  But leaves also contain other chemicals including antioxidant chemicals and one of the chemicals they contain is a family of chemicals called carotinoids which, as the name suggests, are orangey or yellow.  So once the chlorophyll goes away in the leaf then you see that orangey yellow colour. 

As we get towards autumn, because the leaves know that they're senescing – they're getting old, they're going to be lost soon - they reduce their rate of chlorophyll production and that means that there's less of the green pigment in the leaf and therefore, the yellow colour that the green was previously hiding is disclosed and that's why the leaves appears changed colour because the leaves are running out of chlorophyll.  Some leaves also turn a red colour though, don't they, which is nice.  Not all leaves but some species and that's because actually, they make another class of chemicals called anthocyanins.  These are the same things you find in beetroot.  They're a dark deep red colour and they're also a family of antioxidants.  And what the plants do is put those into the leaves to sustain, and support and prevent stress in the leaves as they go towards winter, and that means the plant has longer to scavenge back from the leaf the things that it wants to rescue back into the plant before it dumps the leaf because once you lose the leaf, you're losing tissue, you're losing salts, and chemicals, and potentially, other good for you things. 

So by protecting the leaf with these other anthocyanin molecules for a little bit longer, you hang on to your leaves for slightly longer than you otherwise would