Adult mealworm beetles, Tenebrio molitor L. (Coleoptera: Tenebrionidae), show density dependent polyphenism in adult cuticular colour that is correlated with prophylactic immunity (Barnes & Siva‐Jothy, 2000). The degree of cuticular colour is important in understanding the biology of a number of insect‐related phenomena (e.g. industrial melanism, thermoregulation). In this paper, the extent of variation in cuticular colour in a large outbred population is documented; the rate of darkening under three different rearing protocols including experimental selection for cuticular colour is examined; and the time taken for cuticular colouring to be completed is determined. The results show that cuticular colouring after eclosion is time‐constrained in T. molitor, regardless of whether beetles are reared in large outbred cultures, inbred cultures, or cultures subjected to selection for darkness or lightness. Blacker beetles always darkened faster. The potential reasons for the constraint and the potential costs of rapid darkening are discussed.