There are several chemical elements and compounds that can burn green. Here are some examples:

• Phosphorus: When burned, phosphorus produces a pale blue-green flame.
• Copper compounds: Copper compounds are commonly used in pyrotechnics to produce green or blue-green flames.
• Barium nitrate: Barium nitrate is a common pyrotechnic colorant that can produce a green flame when used with chlorine donors.
• Barium oxalate: Barium oxalate can produce a green flame in pyrotechnic compositions when used with perchlorates.
• Borax or Boric Acid: Borax and boric acid are two boron salts that can produce a vivid green flame when added to a fire.
• Copper sulfate: Copper sulfate is an algicide and root killing agent that can produce pure green fire when dissolved in rubbing alcohol.

Its worth noting that a green flame in a campfire could also be caused by burning wood that has been treated with boric acid to prevent termites or ants.