[MW Giles]

The colour is simple chemistry - same as on a bunsen burner

Blue - lean mixture - complete combustion of fuel
Yellow - rich mixture - incomplete combustion

Flame dampers never give complete damping as there is a trade off between damping of the exhaust and drag, length of damper, back pressure on engine reducing performance, loss of thrust from exhaust gases etc

They could also degrade over time - hours stuck in a howling gale subjected to high temperatures.

Accounts of night fighters indicate that from directly behind you could often find the exhaust glow