The colour it refuses
Why Are Leaves Green?
Chlorophyll absorbs red and blue light and reflects green — so green is the colour it rejects.
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Step 1 of 5
Colour is what bounces back.
An object's colour is the light it does not absorb. A green leaf is green precisely because it is rejecting green light.
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Step 2 of 5
Chlorophyll is tuned to red and blue.
Its molecular structure absorbs strongly at those wavelengths and poorly in the middle of the spectrum, where green sits.
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Step 3 of 5
The captured energy does the work.
Absorbed light energy knocks electrons loose, which is the first step of splitting water and building sugar. Green light contributes very little.
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Step 4 of 5
Other pigments hide underneath.
Leaves also contain yellows and oranges all summer. Chlorophyll is simply so dominant that you cannot see them.
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Step 5 of 5
Autumn is chlorophyll being withdrawn.
The colour was there all along, underneath As days shorten, trees reclaim the valuable chlorophyll before dropping their leaves. The yellows that were always there become visible.
Plants under artificial light often use red and blue LEDs only, which is why grow lights look purple.
The short version
A leaf is green because green is the one colour chlorophyll cannot use and throws back at you. The yellows sit underneath all summer, invisible — autumn is the tree reclaiming its chlorophyll and leaving them on show.
Try it yourself
Shine a red light and a green light on a leaf. Under green light it looks bright; under red, nearly black.