it is the Anti-Halo coating on the back of the film disolving.
It's the anti-halation dye alright, let me be clear about this. But to expand on (and perhaps rectify) two technical inaccuracies that may be present here:
1: It's not really a
layer as such that dissolves. I.e. the film isn't getting any thinner and no actual part of the emulsion or thin film disappears as the anti-halation dye dissolves. It's in fact just that - a colored dye that's embedded in an emulsion layer, and during processing, the dye dissolves - while the actual emulsion layer it was part of, remains in place. This is contrary to an actual layer that is physically removed, as is the case with a remjet backing layer as found on e.g. Kodak Vision 3.
2: The anti-halation dye does not necessarily need to be on the back of the film. I think in the case of Foma, it actually is, though(both their sheet and roll films). Sheet and roll films generally have a gelatin emulsion layer on the backside (i.e. non-image side) mostly to prevent excessive curling as the film dries. During drying, a gelatin emulsion contracts and this would result in warping, which in roll & sheet film is countered by a similar gelatin layer on the backside of the film, so that the effects on both sides sort of balance out. On 135 film this isn't necessary because the film is narrower and curling is therefore less excessive. Back to the anti-halation dye: this can be embedded in the anti-curl back layer on roll & sheet films, but it's also possible (and AFAIK quite common) to embed it in a lower layer on the image side of the film, but below the image-forming layer. The advantage of the latter approach is that it can also be employed on 135 film, and it's at least in theory more effective since it blocks reflections inside the film base (cf. light piping) as well as halation that originates in reflections against the pressure plate or inner septum of the dark slide (sheet film).
The color of the anti-halation dye is variable; i.e. it varies from product to product, and moreover it can be different depending on the conditions. E.g. Foma's anti-halation dye varies from emerald green to a bluish teal depending (AFAIK mostly) on pH. This also explains why the color can be subtly different from one developer to the next.
Hmmm...coincidence? I think not.
That a similar or identical anti-halation dye is used does not mean it's the same product. It can be, but doesn't have to be. Both Ilford and Kodak B&W films can emerge with a magenta tinge from processing - this doesn't make them the same products. Most color C41 films release magenta dyes during processing, and again, this doesn't make Kodak film the same as genuine Fuji film. Etc.
