I'd suggest that even original colours are subject to great variation anyway, according to year, batch and manufacturer ..and so it makes little difference if a car's colour is a standard or custom. Particularly where the blend and mix ratios are known.
Fading (depending on the proximity of industry &/or coast, the climate and quality of light) and yellowing of paintwork changes hues and tones yet again, as to a certain extend does the primer undercoat. Arguably quanties of lead and other additives in those can have a long term effect on colour fastness. Fading of course tend to hap