This study presents the first functional analysis directed at elucidating the cis‐regulatory modularity of genes in Lepidoptera. Colour divergence in this insect order is thought to be the product of the regulatory evolution of genes coding for pigmentation and those controlling them (Jiggins et al., 2017). Although we have considerably increased our understanding of colour divergence, the functional evaluation of the underlying regulatory structures remains poorly understood. Here, we generated several transgenic lines and have provided functional validation of cis‐regulatory modularity in the yellow gene. Colour divergence of insects is based on various pigmentation in several tissues. Pigmentation genes have been found to be used recursively for tissue‐specific pigmentation, and these multiple uses place a pleiotropic constraint on protein‐coding sequence evolution. Based on our results, we suggest that the cis‐regulatory modularity of the pigmentation gene yellow has been a means to bypass pleiotropic constraints and facilitate gene expression and colour divergence.