It just doesn't make sense that this could be a printing error. Why would the error occur only around the spine. The printing equipment doesn't 'understand' what it's printing, it's just one big surface to it. If it had been a printing error, the colour scheme would have been the same for the whole insert, because it's printed all at once, not in segments.
Also, what you can't really see much on that scan but was reported to me is that the edges of the front and back are also paler beige, so they have suffered partial discolouration. You can imagine the cassette on a shelf with the spine exposed to light, and the edges partially exposed at times as the tape may not have always been even with other tapes next to it.
Anyhow, I asked a friend of mine who's into printing and he told me he was 99% sure it was discolouration. When surfaces are exposed to light, the green pigments are always the last to go, and the blue pigments the first ones. So the beige on the spine which includes no or almost no green is now almost white while the brown which includes a good share of green is still visible, in green. This is very likely the explanation behind the alternate colours for "Strangeways...", "Girlfriend In A Coma", etc. as they also leave green but lose other colours in the mix. I'm sorry I simplified his explanation as I can't remember his exact technical lingo, but that was the gist of it.
Of course we'd all want this to be a strange mispressing with some mystical explanation behind it, but I'm afraid it isn't.
for what it's worth,
Stephane