When speakers refer to objects in the world, they often overinform: provide their listener with redundant adjectival information. Contrary to classical theories in linguistics, recent theories have framed overinformativeness as an efficient means of grounding reference in perceptual information of high discriminability to facilitate listener comprehension. However, the generalisability of such theories is constrained by the methodological challenge associated with reliably manipulating the perceptual discriminability of naturalistic stimuli. Here, we overcome these methodological challenges, using methods from psychophysics to manipulate the perceptual discriminability of colour and material attributes in a reference-production experiment. We provide a robust validation of the view that overinformative reference is driven by speakers grounding expressions in attributes of high discriminability. However, we also find that colour information is privileged above and beyond such factors of discriminability.