Should you use paint that comes in "authentic" colours or not ?
I have mixed feelings on the subject. Most of the time I'm too lazy to go to the shop and too stingy to spend the money on buying the real thing. A close-ish match from the Humbrol range will do me thank you very much. Given a coat of grime, only idiot pedants will disagree and face the pointy stick of doom from behind the layout at shows.
One exception is warning yellow. Humbrol produce a perfectly acceptable yellow, several in fact, but none quite looks right. On the prototype it's a slightly green shade whereas the nearest equivalent doesn't have this. To be honest, it's a subtle distinction and the requisite grime will probably hide the "error".
You have also to bear in mind that most train sets aren't viewed in daylight. Mine appear under normal incandescent lights and even if I swapped to daylight balanced ones, the light spill from the rest of the hall negates the effect. Only once have I seen a serious effort to deal with this issue but it came at the cost of a lighting control box the size of the model and even then couldn't defeat the sunshine through the windows facing the display.
Anyway, what I'm saying is that the yellow used is from Railmatch whereas the red buffer beams are Humbrol. Not quite right but I've never modelled a clean bufferbeam and don't plan to start now.