The relationship between propellant chemical composition and the color of a rocket exhaust flame is a basic physical and chemical principle that does not require citation.
The claim states that the orange flame color of certain rocket engines results from the combustion properties of its propellant mixture (such as the presence of specific metal species or carbon emission). This is a well-known scientific principle regarding emission spectra and combustion chemistry, rendering it common knowledge that does not require specific paper citations from the provided literature.