Colouration is important in mate recognition and selection. The ‘species recognition hypothesis’ predicts that sister species with overlapping ranges will have greater colour pattern divergence. We tested this hypothesis with brightly coloured coral reef butterflyfishes (genus: Chaetodon). Using a novel digital imaging methodology, we quantify both colouration and pattern using 250,000 sample points on each fish image. Surprisingly, evolutionary age did not affect colour pattern dissimilarity, with large colouration differences between sister pairs already established within 300,000 years. However, the effect of range overlap and range symmetry were significant. Sister species colour patterns become more dissimilar with increasing overlap, but only when ranges are similar sized. When ranges are disproportionate, sister species colour patterns become more similar with increasing overlap. Species with small ranges appear to maintain non-colour based species boundaries.