Subtle "champagne" patina on vintage white/silver dials

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More of a general vintage watch question than an Omega one specifically. I've noticed that 1960s white-silver dials sometimes vary subtly in colour even in otherwise identical examples in really good, authentic condition. It seems that some dials have a slighty off-white, champagne-like tint while others (maybe in original conidtion?) remain brighter white/silver.

I'm not talking a really obvious patina or colour change, just a subtle tint. For example, I have a couple of early 1970s King Seikos with the same dial, and one definitely has a slightly more off-white tint, bordering on champagne.

The thing is, I think those champagne-tinted examples are gorgeous! Is it just that there were colour differences in manufacture, or (more likely?) they have acquired patina over the decades? If the latter, what causes it? UV light perhaps, or air pollutants if the case is not airtight?