Grapheme–color synaesthesia or colored grapheme synesthesia is a form of synesthesia in which an individual's perception of numerals and letters is associated with the experience of colors. Like all forms of synesthesia, grapheme–color synesthesia is involuntary, consistent and memorable. Grapheme–color synesthesia is one of the most common forms of synesthesia and, because of the extensive knowledge of the visual system, one of the most studied.