AuAgCU

I emailed the Royal Mint Museum again and brought up the topic of pickling once more. Here's what they replied:
»I have been in contact with the relevant department and I have been informed that although the pickling process has changed over time, this change wouldn’t massively alter the colour of the coin.  Therefore the colour change will be predominantly from the improved metal refining techniques which has reduced the amount of silver present in the coin.  

The colour of the sovereign can also depend on whether it is a bullion or a proof coin.  About 10 years ago we stopped making the blanks for gold sovereigns internally in the Mint, and instead began sourcing them from a company in Madrid.  For bullion coins we would strike them straight out the pack as it were, with no alterations being made to the metal.  For proof coins however, we would burn the surface of the coin to give it a more shiny appearance. 

All of these factors could contribute to the changing colour of the sovereign over time, but as you can imagine the process for making the coins is very extensive and technical and it often depends on each individual coin.  It can be concluded though that the metal refining techniques would be the key reason for the main change in colour of the gold sovereign.«

There is a website called what do they know.
It makes freedom of info requests very easy. You could try there as one last try, if they refuse they need a very good reason.

I think you have solved it thank you very much. 
Your source talks about pickling the blanks in sulphuric acid "before they passed the dies" which is why the process is not evident from pitting and pockmarking, leaving the process detectable only from the colour. Excellent work.
Is anyone willing to chuck one of their harrington and byrne sovs in some sulphuric acid to see what happens? Pockmarked and pitted to hell no doubt, but should be a much nicer colour. 
 

Could it be that the Royal Mint has stopped pickling its coins? According to this old book I found on google books, pickling was part of the manufacturing process of sovereign coins (Ernest Seyd. Bullion and Foreign Exchanges. London, 1868, p. 551)
https://books.google.de/books?id=SL1VAAAAcAAJ&pg=PA551&dq=pickling+gold+coins&hl=en&sa=X&ved=0ahUKEwjDvo_hytzgAhWSzaQKHZv2AtsQ6AEIKjAA#v=onepage&q=pickling gold coins&f=false
Pickling was also used for other coins, e.g. the Swiss "Vreneli", making the coins look more golden than their actual alloy by removing some of the copper from the surface. See C-18) from the following FAQ by the Swiss Mint (German only):
https://www.swissmint.ch/d/downloads/aktuell/FAQ-de.pdf
To find out if my guess was right I just emailed the Royal Mint Museum. They should be able to settle the question. I am planning to post their answer here as soon as they get back to me.

Could it be that the Royal Mint has stopped pickling its coins? According to this old book I found on google books, pickling was part of the manufacturing process of sovereign coins (Ernest Seyd. Bullion and Foreign Exchanges. London, 1868, p. 551)
https://books.google.de/books?id=SL1VAAAAcAAJ&pg=PA551&dq=pickling+gold+coins&hl=en&sa=X&ved=0ahUKEwjDvo_hytzgAhWSzaQKHZv2AtsQ6AEIKjAA#v=onepage&q=pickling gold coins&f=false
Pickling was also used for other coins, e.g. the Swiss "Vreneli", making the coins look more golden than their actual alloy by removing some of the copper from the surface. See C-18) from the following FAQ by the Swiss Mint (German only):
https://www.swissmint.ch/d/downloads/aktuell/FAQ-de.pdf
To find out if my guess was right I just emailed the Royal Mint Museum. They should be able to settle the question. I am planning to post their answer here as soon as they get back to me.

Could it be that the Royal Mint has stopped pickling its coins? According to this old book I found on google books, pickling was part of the manufacturing process of sovereign coins (Ernest Seyd. Bullion and Foreign Exchanges. London, 1868, p. 551)
https://books.google.de/books?id=SL1VAAAAcAAJ&pg=PA551&dq=pickling+gold+coins&hl=en&sa=X&ved=0ahUKEwjDvo_hytzgAhWSzaQKHZv2AtsQ6AEIKjAA#v=onepage&q=pickling gold coins&f=false
Pickling was also used for other coins, e.g. the Swiss "Vreneli", making the coins look more golden than their actual alloy by removing some of the copper from the surface. See C-18) from the following FAQ by the Swiss Mint (German only):
https://www.swissmint.ch/d/downloads/aktuell/FAQ-de.pdf
To find out if my guess was right I just emailed the Royal Mint Museum. They should be able to settle the question. I am planning to post their answer here as soon as they get back to me.