Male nuptial colour hues are important for the maintenance of reproductive isolation among cichlid fish species, and environmental changes that lead to narrower light spectra can lead to hybridization. However, cichlid species can naturally co-occur in narrow light spectrum habitats, such as turbid shallow lakes and the deep benthic zones of African rift lakes. Closely related species from narrow light spectrum habitats tend to differ little in the palette of male nuptial colours, thus for these taxa differences in colour patterns may be more important than differences in colour hue for species recognition. To investigate this hypothesis we examined morphometric and genetic differentiation among males of four sympatric putative species within the deep-water genus Diplotaxodon. These taxa live in a narrow-light spectrum environment where only blue light is present, and males differ primarily in ‘monochromatic’ black, white and silver patterning of the body and fins. Significant genetic differentiation was present among taxa in both microsatellite DNA and mitochondrial DNA, including one pair with no significant morphometric differentiation. Thus, these taxa represent reproductively isolated biological species, a result consistent with male nuptial patterning being important for species recognition and assortative mating. As such, we suggest that narrow-light spectra need not always represent barriers to effective visually mediated mate recognition.