The colour patterning of animal skin is a striking example of how living matter spatially self-organises. In most cases, this process happens during embryogenesis. However, in the case of the ocellated lizard (Timon lepidus), the adult pattern is established post-hatching, thus giving a great opportunity to study the dynamics of skin patterning in vivo. The final distribution of green and black skin-scales arranged in a ‘labyrinthine’ pattern is reached through a series of scale colour switches, which can be modelled in different ways. First, we consider a phenomenological model in terms of a ‘stochastic cellular automaton’ where the skin-scales provide the spatial discretisation. This description gives an interesting link with the Ising model: in this picture the lizard pattern is a frustrated anti-ferromagnet. Second, we consider an effective model of interactions between chromatophores which takes the form of a reaction-diffusion system in a domain with varying thickness, representing skin-scales. In this model, the patterning is the consequence of a Turing instability, and the peculiar scale-by-scale colouring and colour switching is a consequence of the skin-scale geometry. An extra feature of the model is that the precise colour of a green (or black) skin-scale has a subtle dependence on the number of nearest neighbour skin-scales which have the same colour. This prediction could be successfully verified in real lizards.