Supplementary material from "Distinct colours in the ‘synesthetic colour palette’"

In grapheme-colour synesthesia, particular linguistic elements evoke particular colour sensations. Interestingly, when asked, non-synesthetes can also associate colours to letters, and previous studies show that specific letter-to-colour associations have similar biases to those of synesthetes. However, it is an open question whether the colours reported by synesthetes and non-synesthetes differ overall: is there a ‘synesthetic colour palette’? In this study, we visualize the overall distribution in colour space of colour concurrents in grapheme-colour synesthetes, and colour associations in non-synesthetic controls. We confirm the existence of a synesthetic colour palette: colour concurrents in synesthetes are different from colour associations in non-synesthetes. We quantify three factors that distinguish the colour palette of synesthetes and non-synesthetes: synesthetes have an increased over-representation of ‘pure’ (unmixed) hues, an increased presence of ‘warm’ (yellow-orange-brown) colours, and an increased presence of achromatic (grey, white, black) colours. Furthermore, we demonstrate that differences in the synesthetic colour palette can be used to train a machine learning algorithm to reliably classify single subjects as synesthetes versus non-synesthetes without using test–retest consistency. As far as we know, this is the first time an individual could be ‘diagnosed’ as synesthete, based only on his or her colours evoked by letters.This article is part of the discussion meeting issue ‘Bridging senses: new developments in synaesthesia’.