I unwrap airlayer today to see roots and notice foil color .
The darker side of foil was inside and look carefully you can see the ends where the bottle gets shinny on the foil.
I put foil on table to maybe get better picture in less brightness.

The lighter side was outer portion of foil

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Looks like you took that foil from the barbeque grill, that is interesting. I've never noticed that on mine

I doubt it is from heat. Probably a chemical reaction between the aluminum and the material encasing the air layer.

Looks like moisture build up. No cause for concern. If you hold a peace of foil over a tea kettle or anything with steam it will do the same thing.

Yes no i do not worry about it . lol
Its a reaction of some sorts and nothing else.
Just thought i would show this cause its not been posted before.

anyone's air layers with foil baking from the heat?  just curious if i should even start one when it's 95+?

It is probably from the fertilizer salt runoff collecting in that area. Acidity will also cause the oxidation reaction, which is why tomatoes are not to be cooked using aluminum foil.