Hope I am not rambling too much but this thread really got me thinking.

If I can't get the correct colour code I colour match by eye eg get the part get a close colour and then tint untill it's right. Explanation below

If you are using old colour code books you may have problems gettting the colour to be right. Some tinters have been obsoleted and tinter strenghts have changed dramatically (acrylic lacquer) so the colours not going to be right but it will be a good starting point or sometimes spot on. Sometimes if you are painting the whole interior it doesn't matter as you have nothing to compare it with.

Another reason why the colour might not be right is that in the factory there was always some variation in the tinting. When I worked as a Panelbeater/Spraypainter at a Ford dealership 14yrs or so years, sometimes we would have two identical cars in at the same time both new. Both the same colour until you parked them up against each other. Sometimes manufacturers also change suppliers as well.

Heres where I really got to thinking and I will have to do some more research on this one but theres a place here in WA that has what I think is called a spectrometre. It analysis the colour sample then gives the right tinters to formulate the colour, using todays tinters. If it pans out it could be the way to go providing you have agood sample.