The conspicuous colours of many sit-and-wait predators are known to visually lure prey. These displays are often polymorphic, but almost nothing is known about the adaptive drivers of variation in this context. Here we explore the sensory-drive hypothesis that variation in light environments and/or receiver sensory biology may favour polymorphic signalling solutions. We conducted an observational study of the orb-web spider Gasteracantha fornicata that has discrete (human-perceived) yellow and white morphs. We combined data on light environments and prey captures collected at fine temporal scales with visual modelling, to explore the chromatic and achromatic conspicuousness of spiders from the perspective of their predominantly fly prey. Our results show that at the times at which prey are captured, yellow individuals are disproportionately chromatically contrasting against their backgrounds (compared to all other times of the day), whereas white individuals show disproportionate achromatic contrast when capturing prey. Further modelling suggests that yellow and white signals are designed to enhance chromatic and achromatic conspicuousness, respectively, in the heterogeneous light environments that G. fornicata inhabit. Our results suggest that colour-lure morphs may have arisen as alternate strategies for maintaining signal efficacy in variable environments by targeting different visual channels in prey.