LOX and RP-1 never burn absolutely clean, and there is always a bit of free carbon in the exhaust, which produces a luminous flame. So when you’re looking at TV and see a liftoff from Cape Kennedy—or from Baikonur for that matter—and the exhaust flame is very bright, you can be sure the propellants are Lox and RP-1 or the equivalent. If the flame is nearly invisible, and you can see the shock diamonds in the exhaust, you’re probably watching a Titan II booster burning N2O4 and 50–50.
Clark, John D. Ignition! An Informal History of Liquid Rocket Propellants. Rutgers University Press Classics, 2017. p. 96