Airlock stained with bronze color during primary

Curious if anyone has experienced an airlock turning brownish or bronze color during fermentation. I use Star San solution in airlock during fermentation. Have only noticed this from a couple of yeasts and both are known for producing sulphur smell during early fermentation.
I figure it is the caused by the mixing of sulphur gas (possibly dioxide) being produced early on in primary, and acid from Star San in airlock and producing higher acid content in airlock.
BTW, have only observed this in California V, and German / Kolsch 029. As I mentioned, both known for sulphur gas production during early fermentation.

Did wort overflow/bubble into the airlock during fermentation? That would be my guess.

No. Normal kreusen of around two inches. Agree, would have expected that if overflow occurred but not near top.