Toning Like A Ticking Clock...

Fortunately, this isn't a particularly pricey coin, so I'm not too bothered. So far it's gotten better. But I expect in 15 years or less, it'll be turning black.

TPG holders are not air-tight, nor are air-tites. Plastic simply isn't a barrier to gasses. Some is better than others (mylar would have better archival properties, and metalized mylar better still).

You could store coins in a vault under a vacuum in extreme cold to slow chemical reactions as much as possible, or you have them on your shelf and enjoy them.

The lighting really isn't all that different. Same camera, same lighting diffusers, same lights (jansjos), just a bit more light from the bottom and somewhat different exposure settings. You can also look at the photos from the original HA sale. There was only blue on the rim and, not all over the cap and the field.

I'd crack it out and rinse it in acetone. That should stop the oxidation. Then put it in a Mylar flip and it should not change for many years.