It won't have a RAL (or BS4800) code as it was not created as one of those series though, there may be a RAL colour which is close.
Litchfield green is Triumph code 45 - which is PPG (paint manufacturer) code 42464 according to Paul Teglers website
BS4800 2011 colourshttp://www.e-paint.co.uk/BS_Colourchart.asp
Both websites randomly selected......
Most decent paint suppliers have sophisticated colour matching equipment that cross references to standard codes, so if you have small, unweathered component you can take in it might be worth a try. I know that when I get matched paint for my Vitesse it comes back with a BS4800 number code even though it was originally a Rover colour.
Litchfield green is a nice colour and I just seen a pic of a Mk2 GT6 that looks superb in it. Now wondering if it could suit a Mk3.......