The BMIHT certificate will not break down the trim colours beyond the basic statement: Trim- Grey.
If you have a Pacific Green car or a Florida Green car, the grey trim will always have dark green carpets and dark green piping on seats, armrest, dash top outer edges, rear squab, etc.
Heritage Upholstery offers a colour that's close but a bit too light, called Parchment.
Another colour they have over in their German trimming materials that's about right on for the Healey "grey" is called Porche Linen. Get them to send samples.
Another point, if you have a BN6 or BN7 trimmed in grey, the Armacord in the interior was substituted with a tan coloured Hardura, which is a Jaguar colour and material which Heritage also can supply.