Colour plays an eminent role in beadwork. Colour modifications are reported on early shell beads from Middle Stone Age sites. However, identifying the colouring agent and demonstrating the intentional nature of the colouring process is not straightforward. Here, we provide analytical data on colour and structural modifications observed on Nassarius kraussianus (Nk) collected in modern thanatocoenoses and on shells of the same species experimentally heated in oxidizing and reductive atmospheres. Comparison with Nk shell beads from the 72 ka old Middle Stone Age levels of Blombos Cave, South Africa, and contextual analysis of other malacological remains from the same levels that were not used as ornaments identify the mechanisms responsible for the change of colour in modern Nk thanatocoenoses and heated shells, and show that although some Nk shell beads were heated, intentional heat treatment of shell beads is not demonstrated.