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Abstract Insect colouration mediated by melanization can assist in dealing with environmental temperatures. However, melanin synthesis can be costly and depends on the ability of insects to acquire enough energy and nutrients from their diets. Due to the increased plant C:N ratio associated with elevated CO₂concentrations, insect herbivores' melanization could be limited by the amount of nitrogen they acquire from their host plants.To investigate how diet C:N impacts the potential colour response to temperature, we usedManduca sextacaterpillars reared at different combinations of temperatures and diet C:N ratios, and measured pupal mass and development time (performance metrics) and colour morphology.The high‐temperature treatment (27°C) had a positive impact on larval performance, whereas a nitrogen‐poor diet was related to lower performance. Using a fitness metric that considers both pupal mass and development time, we found a positive effect of both high temperature and nitrogen‐rich diet treatments on larval fitness.We found that diet and temperature affected the colouration of larvae, in which larvae reared at the low‐temperature treatment (18°C) and fed a nitrogen‐rich diet were darker than their counterparts.Our results provide experimental evidence of the impact of diet on melanization and suggest that CO₂‐related changes in plant quality could be associated with changes in insect herbivore performance and colouration.