There is indirect evidence that categorical colour perception (better discrimination of colours from different categories than those from the same category) can be learned. For instance, CP can be induced across a newly learned category boundary (Özgen & Davies 2002). Here we replicate and extend Özgen and Davies's category learning study to try and pinpoint the nature of the changes underlying category learning. Participants learned to divide green into two new categories 'yellow-green'/'blue-green' across four days. The trained group showed CP across the new boundary on a target detection task and this was restricted to the left hemisphere (LH; cf. Drivonikou et al. 2007), whereas the controls did not. The results could suggest that category training produces changes at early stages in visual processing mainly in the LH. © 2011 - John Benjamins B.V.