For both adults and infants, whether a unique target pops out from a background of distractors and captures attention depends on the perceived target-distractor similarity. For adults, this similarity can be categorical as well as perceptual, but whether the same is true for infants is unknown. Here, the authors examined whether infants’ colour pop-out is affected by categorical target-distractor similarity using targetdistractor pairs in Munsell colours from either the same or a different category and representing different degrees of perceptual similarity. It was found that only targets that were perceptually most dissimilar from the distractor colours popped out, irrespective of category membership; target colours more perceptually similar to the distractor colour did not, even if they were from a different colour category. Although young infants are known to categorize colours in an adult-like fashion, whether or not they preattentively detect a uniquely coloured target is determined by a noncategorical, perceptual colour metric.