Cultural relativists adduce the variation in colour categories across languages as prima facie evidence for linguistic relativity (language affects thought). However, there have been very few experiments that have gone beyond this observational level to assess the extent and the nature of linguistic differences on colour categorisation and perception. Here, we report experiments comparing English and Turkish speakers using a colour-grouping task and same - different tasks aimed at redressing this lack. Turkish categorises the blue region with two basic colour terms (lacivert ‘dark blue’ and mavi ‘blue’) whereas English has a single basic term. In experiment 1 subjects sorted a representative set of 65 colours into groups on the basis of their perceptual similarity. Native Turkish speakers were significantly more likely than native English speakers to form two distinct blue groups corresponding to the two basic blue terms of Turkish. In the same - different tasks we sought for possible categorical effects: enhanced discrimination across category boundaries and/or reduced discrimination within categories. For successive presentation, Turkish speakers were more accurate than English speakers in judgments of colour pairs that fell on opposite sides of the lacivert - mavi boundary. However, for simultaneous presentation, there was no difference between the two language groups. The results suggest that there are detectable effects of linguistic categories on colour cognition, but the locus of the effect may be in memory rather than perception.