The idea that the fitness value of body colouration may be affected by biochemically mediated trade-offs has received much research attention. For example, melanisation is believed to interact with other fitness-related traits via competition for substrates, costs associated with the synthesis of melanin, or pleiotropic effects of the involved genes. However, genetic correlations between colouration and fitness-related traits remain poorly understood. Here we present a quantitative genetic study of a colouration trait correlated to melanin-based cuticular darkness (‘darkness’, hereafter) in a geometrid moth, Ematurga atomaria, which has considerable variation in larval appearance. We focus on correlations between larval darkness and fitness-related growth performance traits. Both a half-sib analysis and an ‘animal model’ approach revealed moderately high heritabilities of larval darkness and indices of growth performance. Heritability estimates of darkness derived from the animal model were, however, considerably higher than those based on the half sib model suggesting that the determination of colouration includes genetic interactions and epigenetic effects. Importantly, on the host plant with the largest sample size, we found no evidence for either genetic or environmental correlations between darkness and growth parameters. On an alternative host plant, there was some indication of positive genetic and negative environmental correlation between these traits. This shows that respective relationships are environment-specific. Nevertheless, the overall pattern of weak and inconsistent correlations between larval colouration and growth parameters does not support universal trade-offs between these traits, and suggests that physiological costs of producing colour patterns do not necessarily interfere with adaptive evolution of colouration.