So what other species fade, especially when kept entirely in the dark?

In 1997 I collected one specimen of Exilia expeditionis (Dell 1956),
which was a deep mauve-maroon. It has been kept since in a closed
dark map-cabinet in a room with thick curtains drawn 24/365 and in
which lights are rarely turned on. Yet it has faded to a light peachy
mauve. If I had known this would happen I'd have had color
photographs taken when it was fresh, as this was an oddball color.

Paul, your explanation for fading certainly applies in many cases,
but not all. Some pigments really do fade quickly, especially some
reds and yellows. But why they should fade in the DARK is beyond me.

As an aside, Surculina and Benthovoluta have been synonymized with
Exilia by Kantor, Bouchet & Oleinik 2001 in "A revision of the Recent
species of Exilia, formerly Benthovoluta (Gastropoda:
Turbinellidae)", Ruthenica 11(2): 81-136. They also synonymize
Benthovoluta gracilior Rehder 1967, B. claydoni Harasewych 1987 & B.
prellei Bozzetti 2001 with E. hilgendorfi (von Martens 1897).