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Right before Falcon 9's engines start, there's a green flash. I assume it's some kind of chemical ignition.

What makes the flash the color green?

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The Merlin engines are ignited with a mixture of triethylaluminium and triethylborane (TEA-TEB); according to Wikipedia:

Triethylborane is strongly pyrophoric, igniting spontaneously in air, burning with an apple-green flame characteristic for boron compounds. Thus, it must be handled and stored in nitrogen or argon.

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