Hypselodoris cerisae vs H. krakatoa — what's the difference?

I was asked: Hypselodoris cerisae and Hypselodoris krakatoa look very similar — what tells them apart? Going back to the paper, it actually says, clearly:

The colour pattern of H. cerisae is similar to that of H. krakatoa Gosliner & Johnson, 1999, but differs in several consistent respects. In H. cerisae, the body has much more pink to purple pigment, whereas in H. krakatoa the notum is suffused with patches of burnt orange. Both species have prominent purple pigment on the anterior margin of the mantle, but in H. cerisae the posterior end is also purple, whereas it is burnt orange in H. krakatoa. The body appears lower and wider in H. cerisae than in H. krakatoa, but in H. krakatoa the mantle margin is more extensive laterally, overhanging the lateral edges of the body. There are seven to nine gill branches in H. cerisae and only six or seven in H. krakatoa. The mantle glands are distributed in a similar manner in both species, where they are largely uniformly spaced around the entire mantle margin. In our molecular analysis (Fig. 35), H. cerisae is sister to a clade that includes H. jacksoni Wilson & Willan, 2007, H. reidi Gosliner & Johnson, 1999 and H. iba sp. nov. Additionally, H. krakatoa and H. reginae Marcus & Marcus, 1970 form a trichotomy with the clade that includes H. cerisae. All members of this largest clade have a gill peduncle that is well elevated from the notum.

— Reading between the lines: revealing cryptic species diversity and colour patterns in Hypselodoris nudibranchs (Mollusca: Heterobranchia: Chromodorididae)

Translating that into a one-shot field rule:

krakatoa: the posterior end of the mantle is orange.