What gives leaves their green colour?

Chlorophyll, the molecule that does the actual work of photosynthesis. It absorbs light strongly in the red and blue parts of the spectrum and hardly at all in the green, so the green is what bounces off and reaches your eye. Leaves are green because green is the part of sunlight plants find least useful.

The molecule is built around a magnesium atom held in a flat ring of nitrogen and carbon. Haemoglobin, which carries oxygen in your blood, is built to almost exactly the same pattern with an iron atom in the middle instead - two of the most important molecules in biology, differing at the centre.

Autumn colour is chlorophyll leaving rather than anything arriving. It is expensive to maintain and the tree dismantles it before dropping the leaf, reclaiming the nitrogen and magnesium; the yellows and oranges underneath were there all summer, masked. The reds are different - many trees actively manufacture those pigments in autumn, and why a dying leaf would be worth investing in is still argued about.