Nepenthes is a genus of carnivorous pitcher plants with high intra- and interspecific morphological diversity. Many species produce dimorphic pitchers, and the relative production rate of the two morphs varies interspecifically. Despite their probable ecological importance to the plants, little is known about the selective context under which various pitcher traits have evolved. This is especially true of colour-related traits, which have not been examined in a phylogenetic context. Using field observations of one polymorphic species (N. gracilis) and comparative phylogenetic analysis of 85 species across the genus, we investigate correlations between colour polymorphism and ecological factors including altitude, light environment and herbivory. In N. gracilis, colour does not correlate with amount of prey captured, but red pitchers experience less herbivory. Throughout the genus, colour polymorphism with redder lower pitchers appears to be evolutionarily favoured. We found a lack of phylogenetic signal for most traits, either suggesting that most traits are labile or reflecting the uncertainty regarding the underlying tree topology. This study highlights ecological correlates of the vast phenotypic diversity of this group of tropical plants. We point to a need for future study examining herbivores of Nepenthes and experimental investigations on colour polymorphism.