I remember two red cores and earth, think there was a three red core and earth, but when the colours changed have not seen twin brown or three brown and earth although they would be the correct colours, but having the phase colours even if the installation is single phase does allow one to follow the colours switch to switch although all should have brown or red sleeves.
In the UK we had red, yellow, blue, in rest of Europe it was brown, black, black, when we harmonised and we got the grey wire not sure if it helped or hindered the European electrician as they had not bothered with keeping the phase rotation, and I know I have been caught out using a phase rotation meter when the rest of the factory was wrong.
But you can't have a colour code for the correct three brown wires, they are all the same colour, so when one uses the wrong colours you clearly also can't have a colour code, personally I am glad we use three colours for two way switching, it does make it easier, but not sure on using blue for the switch wires, so many people jump to the conclusion it must be neutral when its not.
Fitting sleeves or numbers and letters is all well and good when using ferrules so they can't fall off, but without the ferrule one must realise they will fall off unless shrink is used, and domestic electricians don't tend to have hot air guns or ferrule crimpers in their tool kit.