Red

The colours of autumn leaves are caused by flavonoids which are pigments created in the leaves when the temperature drops. The flavonoid compound which is responsible for red, blue, purple or magenta pigment is called anthocyanin. Yellows and oranges are due to carotenoids which are present all year round but are masked by the larger amounts of chlorophyll, which is green.

That's a bit of science for you. I've been doing so much reading and studying today that I think I might be at risk of Mad Brain Fever - I think I'll calm it with Midsomer Murders later