Visual signals are used by animals to meet a range of communication purposes, including: to attract mates, defend territories and to deter predators. The sensory and psychological traits of signal receivers can act as selective pressures that drive the evolution and design of visual signals. Here, we investigated whether particular components of conspicuous warning signals (colour, pattern, borders) improved learning that a prey item was unprofitable by marine predators, and also which cues they paid most attention to when making generalisations about novel visual stimuli. To do this, we conducted behavioural experiments in aquaria with our model fish predator, the Picasso triggerfish (Rhinecanthus aculeatus). Fish were trained to approach and tap at different coloured stimuli to either receive a palatable or unpalatable food reward. Visual stimuli were designed based on those displayed by nudibranchs (sea slugs). These shell-less molluscs use conspicuous colours and patterns to warn predators they contain toxic compounds, which they sequester from their sponge diet. We designed our coloured stimuli based on Chromodorididae nudibranch colour patterns, many of which have a yellow border, with red or blue highly contrasting internal patterns. We found that the external yellow border strengthened avoidance learning by predators, and this was also an essential component of the signal for fish to make generalisations between learnt colour patterns and novel (but similar) colour patterns.  These results help explain variation within internal patterns of nudibranch signals both within and between species, as avoidance learning seems to be achieved through the highly conserved yellow border. This study provides intriguing insights in to the importance of pattern in animal visual signals to predators, the relative role of each signal component, and how variation in warning signals can be maintained in the marine environment.