The Triumph TR6 was popular during a very colourful time in history, and many were painted with fanciful colour schemes or bright colours. As with most things vintage, restoring your TR6 as close to factory specifications will probably yield the greatest market value for your efforts.
Original Color Codes
Your TR6 may have the original commission plate attached to a front bumper and visible from the engine compartment, or attached to a door jam. This plate will show the original colour codes for the vehicle. If you are doing an historical restoration, use an old colour chart to come as close to the original paint and trim colour as you can.
Colour Charts or Scrapings
Colours will have faded on physical paper charts, and scanned ones may not be accurate, but they are still a good starting reference. If your TR6 has never undergone a total restoration, you may be able to find sections of the original paint and trim that have been covered and protected from sun and weather exposure since the car was first painted. Scrape off sections and either have them scanned for colour or compare them to contemporary colour chips.
Local or national chapters of Triumph clubs will have a wealth of information about restoration of Triumphs of all kinds. Check out online forums or post your own questions before embarking on your TR6 painting project. The clubs will have good information about the different paint schemes that were availed for different models and years.