Species colour has multiple ecological functions, such as camouflage, signaling, UV protection, and thermoregulation. Macrophysiological patterns are crucial to understand colour adaptation. Thermal melanism hypothesis suggested that darker ectotherms are favored in cold area, due to higher solar absorbing rate. UV protection hypothesis suggested that species with dark cuticle could reduce cell damage from UV radiation. Melanised cuticle may also increase species’ pathogen resistance, favored under warm and/or humid area. Although the thermal melanism hypothesis obtained widely empirical supports, these competing hypotheses have rarely been tested for tropical ectotherms, for which heat absorbing may be less important. We investigated colour lightness of butterfly assemblages appeared in various climatic conditions from 1993 to 2014 in Taiwan. We considered temperature, solar radiation, precipitation and their two-term interactions as predictors for butterfly colour lightness. Our model showed weak support for thermal melanism hypothesis but the butterfly assemblages were darker with high solar radiation, in agree with prediction of UV protection hypothesis. Given the high correlation between temperature and solar radiation, we proposed alternative explanation that darker butterflies may prevent themselves from overheating by rapid cooling rates. Our study suggested that color-thermal adaptation in the tropics is complicated than previously thought and integrated macrophysiologial experiments are required to reveal the underlying mechanism.