Colour polymorphism has been previously reported in several colubrid snakes including Boiga spp. In this paper, we report colour variations within the poorly known southern Indian Boiga dightoni, provide the first molecular data for this species, from two localities (including the type locality) and compare them with data from other congeners. Additionally, we provide detailed dentition and hemipenis descriptions for B. dightoni. Molecular data for B. dightoni show very little difference (0.2–0.4% 16S; 0.9–1.2% cyt b) to the recently described Boiga whitakeri, also from southern India. We have re-examined and present new information on the pholidosis of the type specimens of B. whitakeri and reconsider its taxonomic status. On the basis of molecular data and overlapping morphological characteristics, we argue that Boiga whitakeri and Boiga dightoni are conspecific, and place B. whitakeri under the subjective synonymy of the latter. Furthermore, we show that colour polymorphism in B. dightoni is a gender-independent character and that both colour morphs are found in high as well as low elevations and partly in sympatry. A revised key to the Boiga ceylonensis complex is provided.