Variation in the conspicuousness of colour patterns is observed within and among defended prey species. The evolution of conspicuous colour pattern in defended species can be strongly impaired because of increased detectability by predators. Nevertheless, such evolution of the colour pattern can be favoured if changes in conspicuousness result in Müllerian mimicry with other defended prey. Here, we develop a model describing the population dynamics of a conspicuous defended prey species, and we assess the invasion conditions of derived phenotypes that differ from the ancestral phenotype by their conspicuousness. Such change in conspicuousness may then modify their level of mimicry with the local community of defended species. Derived colour pattern displayed in this focal population can therefore be either exactly similar, partially resembling or completely dissimilar to the local mimicry ring displaying the ancestral colour pattern. We assume that predation risk depends 1) on the number of individuals sharing a given colour pattern within the population, 2) on the occurrence of co-mimetic defended species and 3) on the availability of alternative edible prey. Using a combination of analytical derivations and numerical simulations, we show that colour patterns that are less conspicuous than the ancestral one are generally favoured within mimicry rings, unless reduced conspicuousness impairs mimicry. By contrast, when a mutation affecting the colour pattern leads to a shift toward a better protected mimicry ring, a more conspicuous colour pattern can be favoured. The selected aposematic pattern then depends on the local communities of defended and edible prey, as well as on the detectability, memorability and level of mimicry of the colour patterns.