Space Battleship Yamato 2199, Cosmo Reverse Ver. [WIP – Burnt Metal Effect on the Exhaust Nozzle]

In a move based more on flexing my creative license rather than mimicking physical reality, I went ahead to paint a burnt metal look on the Space Battleship Yamato Exhaust Nozzle. This effect is often found on titanium car exhaust mufflers with a burnt tip (see background image in photo below). While I've seen such burnt effects on jet engine nozzles, I don't have the requisite knowledge to know