Grapheme-colour synaesthesia is a neurodevelopmental condition wherein perception of numbers and letters consistently and involuntarily elicits the concurrent experience of a colour photism. Accumulating evidence suggests that heterogeneity in the visuospatial phenomenology of synaesthesia is attributable to the operation of top-down processes underlying photisms experienced as representations in associator synaesthetes and bottom-up processes subserving photisms experienced as spatially localized in projector synaesthetes. An untested corollary of this hypothesis is that bottom-up mechanisms will actuate earlier photism perception in projector synaesthetes. We tested this prediction in a pre-registered study in which associators and projectors completed adaptive temporal order judgment tasks for graphemes, colours, and photisms. In corroboration of the hypothesis of differential photism access across visuospatial phenomenology subtypes, projectors displayed earlier photism colour thresholds than associators whereas the two subtypes did not significantly differ in veridical colour thresholds. Synesthetes did not differ in grapheme or colour thresholds relative to non-synesthete controls, contrary to previous suggestions of superior colour processing in this condition. These results are consistent with the proposal of differential neural mechanisms underlying photism perception in subtypes of grapheme-colour synaesthesia and warrant renewed attention to heterogeneity in the mechanisms and phenomenology of this condition.