Injectors on full size engines are not painted at all.

It’s like brake calipers on “performance” cars – makes them go faster!

I recall seeing some red injectors on road engines, that’s fine to copy, but not on standard gauge, main line steam railway locos.

The habit is also sometimes seen as an expression of builder’s preference with red levers and controls on the model railway loco backhead. Prototypical for some regulator handles but little else.

Many reasons why parts are painted, to:

1. look pretty (very common on steam engines)
2. cover up flaws, (paint and putty is very common on cast-iron castings)
3. prevent corrosion
4. highlight danger, parts likely to move, get hot, high-voltage etc
5. quickly identify parts for assembly, maintenance & diagnostics

Don’t expect consistency unless colour is done to a standard.   For example, most wiring harnesses are multi-coloured because it makes assembly and tracing bugs easier, which makes sense to me.  However, when I was a lad, British military electronics used pink wire for everything.  Until the item was modified that is, after which pink wire was strictly forbidden.  The reason was that using a different colour to show each change makes it much easier to spot alterations, and alterations are always suspicious! As we all know, everyone apart from you and I is a clown, so good news if colour coding shows where the clot has been.   Makes extra sense in the military, where repairs and mods were done on a ship or in the field under trying conditions, with the equipment returned to be refurbished by a fully equipped facility later.

I don’t know if the pink wire system is still used.  May be obsolete because it’s so difficult for ordinary folk to mend multi-layer printed circuit boards packed with SMD, ASICs, FPGA, and microcontrollers.  Easier to swap boards and send the old one back to a specialist or to bin it.

Dave