The bluestreak cleaner wrasse, Labroides dimidiatus, and the bluestriped fangblenny, Plagiotremus rhinorhynchos, display bright colors due to photonic structures present in their integuments, inside cells called iridophores. The blue areas of the two fish species as well as the white parts of the cleaner wrasse were optically characterized by spectrophotometry and morphologically by transmission electron microscopy. The observed photonic structures were modeled by periodic bilayers composed of guanine and cytoplasm and were identified as the origin of the blue color of the fishes. Although similar in nature, the iridophores of the two fish exhibit many differences, responsible for the differences observed in the measured reflectance spectra. Visual perception models were used to quantify those colours in order to determine whether they could be discriminated by different fish species of the same ecosystem.