We report a cross-cultural study of colour grouping carried out as a test of the Sapir-Whorf hypothesis (linguistic relativity theory). Speakers of English, Russian and Setswana-languages that differ in their number of basic colour terms, and in how the blue-green region is categorized--were compared on a colour sorting task. Informants sorted a representative set of 65 colours into groups so that members of the groups looked similar to each other, with no restriction on the number of groups formed. If linguistic relativity theory is true, then there should be reliable differences between the three samples in the composition of the groups they formed associated with the differing positions of colour category boundaries in the languages. The most striking feature of the results, inconsistent with linguistic relativity theory, was the similarity amongst the patterns of choice of the three samples. However, there were also significant differences amongst the samples. Setswana speakers (who have a single basic term for BLUE or GREEN) were more likely to group BLUE colours with GREEN colours than either English or Russian speakers. But Russian speakers (who have two basic colour terms for BLUE) were no more likely than English speakers to group light and dark BLUE separately. In addition there were general structural differences in grouping among the samples: they differed in the level of consensus in grouping, the number of groups formed and in the distribution of the number of colours placed in a group. These structural differences may reflect differences in the availability and salience of the colour categories across the languages. Our data support perceptual universalism modulated by weaker linguistic effects.