In some species, male traits are more exaggerated than female traits. To understand the evolution and maintenance of such male traits, it is important to clarify their function in contexts of both intra- and intersexual selection. However, information about the function of specific male traits in both male–male competition and female mate choice remains limited. Male Puntius titteya assume a bright red, carotenoid-based coloration over the whole body. A previous study revealed that females in this species prefer to mate with redder males. In the present study, we examined the function of male coloration in male–male competition under both white and green light conditions. Under white light, in which males could perceive red coloration, males with higher color saturation compared to their opponents became dominant. On the other hand, under green light, in which males could not accurately perceive red coloration, the effect of male coloration on male–male competition disappeared, and only male body size significantly influenced intermale competition, i.e., larger males were dominant. These results indicate that, under environments in which red coloration can be assessed by males, body coloration affects male–male competition. These findings suggest that carotenoid-based coloration in P. titteya males plays a key role in both intra- and intersexual selection.