I recommend the original BRG color provided by Jaguar. Purchase the original paint swatch off of Ebay (one is listed on Ebay Motors that expires in 4 days -- see Ebay Motors - Original Dupont Color Chip Pages (2) 3/80 Jaguar XKE XJ6 XJ12 XJS). Usually these swatches carry paint codes.
- A good paint shop should be able to match the correct BRG swatch.
I don't see a compelling reason to use a metallic (opalescent) green, as this would not be correct for the S3 E Type.
Good luck!