If you are in the UK Polly Palmer @ Bri-Tie Motorcycles in Wales has correct colour matches for sale.
Now as to the "correct" colour. It is a pile of puss, there is no such thing once BSA stopped using viterious enamel.
The paint for each day was mixed by the day shift foreman by hand for the scheduled production on that day with the available measuring material of the day so no two batches were absolutely the same.
Next, the parts were sprayed by hand by a team of spray painters and no two painter will spray parts to identical colours.
Add to that the paint store was not accessible by the night shift so if they were running short because one of the painters was spraying a bit too thick, then the mix got thinned a bit to finish the run.
Our oldest riding member bought the first A10 plunger in NSW ( in Australia also he thinks ) and one of his favourite stories is how when he went to pick up his bike. He refused the one that the dealer had prepared for him. He made them swap all of his accessories from one bike to another. The reason ? Out of the first batch of 14 Golden Flashes there was only 1 that was the same colour gold from one end to another . On some he reckons that every part was a different shade gold so if you had one of them then you could make 7 different "original" Golden Flash golds depending upon which "original" part you picked to match.
Now the first production run was overpainted with gold on black which is why they have a greenish tinge as it was to be a limited edition colour. When it became a standard production colour then the top coat was over a red, blue or cream undercoat and each one of these combinations yields a different end colour as the base coat shows through the top coat.
Spare parts were all finished black then either despatched in that state for the dealer to colour match at replacement or colour coated at the factory if they were to be sold over the counter for the customer to fit.
While this spiel dose will necessarily help you to find an acceptable colour silver but hopefully it will stop you pulling out your hair while bashing you head against a brick wall trying to obtain some thing that dose not actually exist.