In recent decades, thanks to the use of integrated taxonomy, the traditional recognition of a nudibranch species based on observation and colour pattern variation has become increasingly questioned, mainly due to the presence of cryptic and pseudocryptic species complexes. Individuals with the same colour pattern can be genetically identical, but individuals with different colour patterns may also be genetically identical and this variation may instead represent different life stages. But things can get even more complicated. What happens when the same species changes its colour pattern radically as it ages? Here we present two extraordinary examples in species of the genus Nembrotha based on laboratory observation. Specimens of Nembrotha livingstonei Allan, 1933 and Nembrotha yonowae Goethel and Debelius, 1992 were collected in Mozambique and kept in captivity as long as feeding was possible. The results showed that colour patterns in both species changed over time and that this change was linked to diet. Furthermore, species delimitation analysis and comparison of the uncorrected COI pairwise distances of examined specimens from Mozambique and others downloaded from GenBank confirmed that N. yonowae Goether and Debelius, 1992 is a junior synonym of N. cristata Bergh, 1877. Similar studies with laboratory observations are needed on other species of the genus, as they were described on the basis of different colouration, but integrated taxonomy may show different results.