I think you'll find that 'Oxford Blue' is the base for the Transom colour being 9 parts of Oxford Blue plus one part Black which equals 'Britannia Blue'. As told to me by a painter that painted them at Hamble at the time.
Not sure of the original colour, you may find slightly different colours used as requested by new owners and what paint was in the paint store at the time .I Use Mauritius blue no mixing and you can touch in with good match. I would post an image but not worked out how to yet. Nick