That's interesting! I work completely differently from Warren!
I usually have photos of the real car to work from, and so match the colours to what I see in the pictures. But I rarely brush paint, and use automotive spray cans instead, so I will buy colours to match what I am working on.
If the part is in carbon fibre, I will use say 'Rover Charcoal metallic black' to achieve that effect, or Vauxhall champagne Gold for brake hubs, a nice dull gold colour etc. etc.