Adding the element strontium to a color pyrotechnic mix produces a red flame; copper, blue; barium, green; and sodium for yellow. Too much or too little of the chemicals make significant changes ...

but you don't want your new puppy or toddler getting into the bottle of root killer you're using for its flame-color-changing ...

He heats the copper as hot as he possibly can until it turns a gray color. Brent uses reduction flame to remove oxidation from the piece. When copper is exposed to oxygen, it turns green.