Inter-sexual signals that reveal developmental or mating status have evolved repeatedly in females of many animal lineages. Such signals can have important functions in sexual conflict over mating and can shape sexually antagonistic coevolution. However, we know little about how female signal development modifies mating harassment and thereby shape sexual conflict. Here, we combine phylogenetic comparative analyses of a color polymorphic damselfly genus (Ischnura) with behavioral experiments in one target species to investigate the evolutionary history and current adaptive function of female color development. Most Ischnura species have multiple female color morphs, one of which is typically male colored (“male mimics”). In our target species (I. elegans), males and male-mimicking females express a blue abdominal patch throughout post-emergence life. We demonstrate experimentally by phenotypic manipulations of this color signal, that in sexually dimorphic females, developmentally plastic expression of this signalling trait reduces pre-mating harassment prior to sexual maturity. Across species, this signal evolves rapidly, but in sexually dimorphic females, its origin is contingent upon the abdomen color expressed by co-occurring male-colored females. Our results suggest that cross-sexual transfer and subsequent co-option of an original male trait to a novel female anti-harassment function modulates sexual conflict driven by pre-mating interactions.