As I said, I've yet to see a gold one that convinced me, but I'm certainly open to the possibility. Train-spotter or not, there are others much more obsessed by horn buttons than I am, and I don't claim to have researched the topic exhaustively. What I do know is that there is only a single part number for FE-FC horn buttons (7409517), every other model has one and only one horn-button, and I've never seen anything 'official' to explain why there might have been deliberate variations in the colour. I don't deny their existence, nor did I suggest they're "unoriginal".
I know only a little about the decoration process for these things, but I still lean towards variations in tinting being responsible for these odd-ball horn-buttons. After all, if you're working with all three primary colours, it's not hard to produce green (B+Y), violet (B+R) or brown (B+R+Y).
But I can see this could lead into an "Intelligent Design" vs. "Accident of Nature" debate now, so I'll bow out too.
cheers
RET