Sorry Stinkwheel, Charlottes right. Having mixed a 'lot' of colours mainly paints and gelcoats but also specified Pantone, RAL, BS, AFNOR and corporate colours plus creating custom colours etc you use pigments to create colours. The Swedish NCS (Natural Colour System) is in my opinion the best for nailing a colour - its what Dulux used (then left and returned to) and still do. The only thing it does leave out is metallics, pearlescents and the colour shifting paints, but the base tones are effectively covered.
Because pigments are not actually pure there are as many as 35 base colours (in paints) to mix a match. Believe me its not very easy to get a perfect match especially under all lighting conditions. If you get it right under daylight you are doing well.
Just like some colours 'chalk' with UV some plastic pigments will be better than others for durability. Its also why a lot of boat fittings are black - the carbon helps stop UV degradatiion of nylons.