Butterflies have evolved a diversity of colour patterns, but the ecological functions for most of these patterns are still poorly understood. The Banded Swallowtail butterfly, Papilio demolion demolion, is a mostly black butterfly with a greenish-blue band that traverses the wings. The function of this wing pattern remains unknown. Here, we examined the morphology of black and green-blue coloured scales, and how the colour and banding pattern affects predation risk in the wild. The protective benefits of the transversal band and of its green-blue colour were tested via the use of paper model replicas of the Banded Swallowtail with variations in band shape and band colour in a full factorial design. A variant model where the continuous transversal green-blue band was shifted and made discontinuous tested the protective benefit of the transversal band, while greyscale variants of the wildtype and distorted band models assessed the protective benefit of the green-blue colour. Paper models of the variants and the wildtype were placed simultaneously in the field with live baits. Wildtype models were the least preyed upon compared to all other variants, while grey models with distorted bands suffered the greatest predation. The colour and the continuous band of the Banded Swallowtail hence confer antipredator qualities. We propose that the shape of the band hinders detection of the butterfly’s true shape through coincident disruptive coloration; while the green colour of the band prevents detection of the butterfly from its background via differential blending.  Differential blending is aided by the green-blue colour being due to pigments rather than via structural colouration. Both green and black scales have identical structures, and the scales follow the Bauplan of pigmented scales documented in other Papilio butterflies.