well i like to work from a sample of original paint.at work we use a colour catalogue to eye match a colour sample card to the original. if a match cant be found that way we will mix up a colour, spray it on a piece of card/metal and hold it against the bike part with correct colour. we then add tinters as required to come to the correct colour. it does take a little while to do. sometimes we have sent samples to the Spies Hecker paint suppler in Adelaide and they have paint specialist there that can match it by a special method, i think its called spectrum analyisng. we hardly ever do that now and just match it our selves. matching candy colours or metalics is usually pretty easy as there is usually the basic green, blue, red, gold, orange etc and the colurs can be varied by the amount of colour you put on the silver base. with some/a lot of candy metalic colours the see thru colour goes on top a silver base. the final colour will depend on how much of the colour you spray on the silver base. most of the 70's metalic colours on bikes were done like this.
keep in mind that unless the nos fender was kept away from sunlight it wont be an accurate indication of original colour due to fading. i like to find a sample under tank badge for example.
i suggest you look in your Yellow pages for specialist Automotive crash supply place that sells professional quality paints, or look for a specialist motorcycle painter in your area.
another option is to go to your local Autobahn, Motormate, Sprint Auto car accessory shop and they can match and mix up colours for you
generally a paint name like sparkle blue will mean nothing to a painter. it is of no use unless we can know the original colour formula to get some mixed up or cross referenced to a modern brand or can order it from a yamaha dealer by yamaha part number for example. eg with the 74 T500 im doing it is Candy gypsey red which doesnt meant much and its NLA. we have only been able to find a very small sample under some non original black paint behind the oil tank. sometimes you have to carefully go looking for traces of original colours under layers of non original paint others have sprayed over top.