Sepia being composed mostly of mollusc melanin, has the same color as the principal human skin brown pigment.
Authentic sepia ink is prepared from the dried ink sacs of the common cuttlefish, Sepia officinalis. Being a naturally derived organic colorants, the pigment is usually considered as not lightfast. But that's proved wrong as shown in these tests.
Anything but sluggish, cuttlefish are the action painting metaphors that art needed.