Synesthesia, the phenomenon of cross-modal perception, is well known in the visual and auditory realms. Recent studies show that contrary to conventional wisdom there are regular and systematic correlations between smell and vision. Smells have colours, and stronger smells have darker colours. Although learning may play a role in the formation of these associations, there is far stronger evidence for a biological basis of colour-odour synesthesia. Sensory evaluation based on synesthetic principles can optimise product design by harmonising fragrance and colour.