wichita.edu
… various other types of fuel for fires. I distinctly recall encountering as many differently colored flames as there are different types of wood. Brown, light brown, white, black, green, and even yellow flames have all been witnessed by me. It became evident that the color of the flame corresponds to the color and properties inherent in the wood being…
https://www.wichita.edu/academics/fairmount_las/geology/wparcell/Kircher/IterII/index.php