Many insects change colour during ontogeny and, in some, this can be explained by different selective pressures acting on highly dissimilar adult and larval forms. However, some insects, such as many true bugs (Heteroptera), retain similar body shape and share similar predators and habitats through ontogeny. Many of these species also release defensive secretions which are advertised with bright colours, and are thus considered aposematic. In this talk I will present the most recent results on two projects related to this question. We use a comparative approach and predation experiments to understand what drives the evolution of colour changes in butterflies and in bugs.