Warning signals, often in the form of colourful patterns, are used by aposematic prey to indicate unprofitability to potential predators. It is theorized that consistency in pattern is beneficial, so that individuals will be more easily recognized and avoided by predators. However, the pattern of the nudibranch mollusc G. splendida is highly variable with spots ranging in size from large blotches to small spots and in colour from bright red through to maroon and even black. This species sequesters defensive chemicals from a diet of 3-4 different sponges containing diterpenes, and has also been known to vary in the quantity and composition of chemical defenses. Here, we investigate the impact of diet and population genetics on the chemical profile, spot pattern, and spot colour of G. splendida from 5 sites within its geographic range on the East Coast of Australia. We analyze gut contents using 1 H NMR as an indication of sponge diet, and quantify actively sequestered compounds in the mantle using an internal standard. We use spectral reflectance measurements and visual modelling to compare spot colour, and image analysis to compare pattern geometry among sites. Implications for the efficacy of warning signals are discussed.