What makes the chapter keys become gold or silver?

[AzoreanEve]

http://steamcommunity.com/sharedfiles/filedetails/?id=1237264535

Here. First time I started NG+ I noticed my chapter 1 key became gold. I thought it was because I started playing in hard difficulty instead of normal, or because it was NG+.
I used chapter select for other things and thus saw no more changes to the keys regardless of what I did and the difficulty.

Only today I once again played the final cutscene and started a NG+ at hardest difficulty. Surely if I played everything in a row without chapter selects or changing how hard the game was the keys would turn all gold, right? Nope! Finished chapter 1 and 2, and as you can see the second key is still the same.

What are the gold keys tied to then?

[Black Panthera]

Wanna know this too. Any answer in 2019?