AuAgCU

I copy-pasted the email  I received. It actually says "burn" there. I can't tell if that makes sense or if it should really be burnish.

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.«

The Royal Mint Museum finally got back to me. No word about pickling I'm afraid, even though I explicitly asked them about it. All they talk about is the purity of the alloy. Here's what they wrote:

"Over time the Royal Mint has developed its refining techniques as technology has developed, and we are now able to extract almost all of the silver out of the gold that we use in our coins, whereas in the past we were not able to do this so effectively. Therefore gold sovereigns in the past were a brighter colour due to this higher level of silver in the gold."

We know from the book I quoted that gold sovereigns where pickled in the 19th century. I would still very much like to know whether they gave it up (which I still believe) and how that affected the colour of the coin. However, I am not going to ask them about it. I have a feeling they don't want to disclose an awful lot of information about the manufacturing process of their coins, and I don't want to pester them.

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.