A Metallurgical Question

[sidthecat]

I've collected a number of old ringtops and I notice that the gold New York Ideal nibs have a rainbow coloration over all or part of their surface; one Canada nib has it over the lower tines, but not the later nibs.

It looks like a reaction to the ink I'm using (mostly Bungbox Eel - I'm besotted with the color) and I wonder if it indicates that Waterman slightly changed its alloy when it applied for its patent.

Any guesses what's going on?