Food sources of the European abalone Haliotis tuberculata throughout its life cycle are still to be clarified in nature. A novel non-destructive method of digital shell color analysis to reveal the diets of European abalone (ormer) was developed in this study. The method was calibrated using ormers reared under experimental conditions in North Western Brittany in 2012 and fed a controlled monospecific diet to define the shell hues associated with various macroalgae (i.e., Rhodophyta, Chlorophyta, and Phaeophyta). General food preferences were established by comparing the shell hue of wild adult ormers and experimental adult ormers. Shell hue corresponds to the color tint in the HSL color space measured on digital pictures of the shell. Experimentally, shell hue values differed according to treatment, with the most yellow-green hue (72°) for ormers fed Saccharina sp. and the coral hue (25°) for ormers fed Palmaria palmata. High variation in shell color of wild ormers was observed according to the sampling site and/or ontogeny. The diet of wild ormers may be related to the abundance of different drifting algae in their respective habitats. Thus, this non-destructive and easy-to-use technique appears to be a promising tool for determining the diet of Haliotis species and, perhaps, other herbivorous mollusks.