The most common type is grapheme-colour synaesthesia where people perceive words, letters and numbers as distinct colours.

BBC: News

From this it was possible to work out which wavelengths of light the planet reflects, and which it absorbs in other words, its colour.

ECONOMIST: Extrasolar planets

This can include changes to the size and colour of fonts, shifting patterns of words and moving backgrounds.

BBC: 'E-poetry' creates electronic sonnets

The idea behind both proofs of the four colour theorem is to suppose that the theorem is violated to assume, in other words, that there is some sort of map that requires five colours to fill in.

ECONOMIST: Just what does it mean to prove something?