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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.

Note that this effect should not be confused with the green flame seen occasionally from damaged Raptor engines that are running "engine-rich", i.e. starting to destroy themselves, and is the color of burning copper. Raptor is spark-ignited, and doesn't use TEA-TEB.

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