The claim that certain anions or elements produce distinct flame colors is a well-established scientific fact taught in introductory chemistry, so no citation is needed.
The claim falls under step 0 as common knowledge: flame tests are a fundamental, textbook chemical observation where specific ions (often metal cations, but also certain species) produce characteristic emission spectra and flame colors. None of the retrieved papers specifically target the detailed chemical phenomenology of anion flame emission tests, making citation unnecessary for this basic scientific observation.