A couple of my charges were a "Sand and Sable" Silver Cloud II, and a "Sand and Garnet" Cloud III.
You're right, the photo doesn't do the colour justice.
Even back then, these colours were just about impossible to get, because when they were new, these Rolls Royce pearlescent paints were formulated with ground-up Mother of Pearl. Consider the economics of having somebody scraping the nacre out of oyster shells by hand...
My best advice would be to try to forego the absolute historical accuracy, and find a modern gold paint that you like, and which is in common enough usage that you can walk into Halford's and get a touch-up pen when you need one.
I had the Gilbern Red paint on my Genie colour-matched with a scanner, and it was very nice, too; however, by the time it came back from the trimmer
Unless the car only comes out of it's storage on high days and holy days, it will pick up scratches and road rash which will need touching up.
Dave