Unfinished Tales - silver vs gilt lettering.

I’m definitely no expert and have no knowledge of how they put gilt on the spines, but the obvious thing I see first is that the middle book has faded cloth compared to the other two. So is this a light damage issue? Do they go direct with gilt on to books or is there an undercoat so to speak, before they finish off with shiny gold?
Obviously this is difficult to see properly on a posted picture so excuse my thoughts. I know you say it’s a definite silver shine.

I think this is just corrosion of the gilt, tbh. Depending on what the gilt is made from, it can change colour in different ways (A green tinge is common due - I believe - to copper oxidising). It is possible that this is different gilt and they loaded up a wrong roll for some copies, but I'd put environmental effects as a more likely possibility, based on the photos. Hard to tell from photos, though.

Thanks both for your comments. I can see that the photos I quickly took on my phone in poor light last night don't really show the colouring properly. I have rectified this below and hopefully this shows the distinction.

As I said in my original post this is does not seem to be a case of tarnished, faded, or oxidised gilt, it is distinctly silver and still shines brightly as silver.

That said my reason for posting isn't really to establish how the colouring came about (whether a case of trial/issue or environment - we can get to that) but because I am curious to see whether anyone else has encountered this lettering on their copies.

Of course, if it were environmentally caused, then there will be many copies out there that look like this. But having catalogued dozens and seen more, this is the first I have encountered.