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Conspicuous animal colouration is predicted to evolve via sexual selection either to increase mating frequency or to reduce unprofitable mating harassment. The selective agents of conspicuous colouration can vary between the sexes and at different developmental stages. The function of conspicuous colouration is well studied in territorial mating systems but poorly understood in non-territorial mating systems. Here, I aim to study the functional significance of males and females conspicuous colouration at different developmental stages in non-territorial damselflies. In ontogenetic colour changing animals, individuals change colour during adulthood but the causes and consequences of conspicuous colouration at different life stages are often unclear. I studied the functions of male conspicuousness in Xanthagrion erythroneurum damselflies. In this species, males but not females carry conspicuous blue bands on the terminal abdominal segments and thoracic colouration of males change from yellow to red during ontogenesis. I performed mating experiments with males before and after colour change and showed that yellow males are sexually immature and attain conspicuous red colouration upon sexual maturity. Then, I showed that male conspicuous colouration (blue abdominal bands and red thoracic colouration) do not increase mating success via female mate choice, but reduce male-male mating attempts and male aggression in breeding territories. By reducing male aggression, conspicuous males can persist in breeding territories, ultimately increasing their mating success. I investigated the conspicuous female colouration in Agriocnemis femina damselfly, where females change colour from conspicuous red to green upon sexual maturity. I showed that males avoid mating with sexually immature red females and preferred green females that are larger and carry eggs. The juvenile females signal their sexual unprofitability with conspicuous colouration, thereby reducing sexual harassment in the pre-reproductive stages. In conclusion, my thesis provides evidence for the selective benefits of male and females conspicuous colouration in non-territorial mating systems.