For some people, letters have colours

A subset of people experience stable colours for specific letters or numbers. The pairings pass consistency tests years apart. This is synesthesia.

Grapheme-colour synesthesia is the common studied form: a printed 5 is blue, or A is red, the same way every time, and the person did not choose it. Consistency tests, given without warning months or years later, separate synesthetes from people who are inventing associations on the spot. Brain imaging finds cross-activation consistent with the reports.

Other forms bind sound to colour, or words to taste. Prevalence depends on the definition. Strong, conscious, consistent synesthesia is uncommon, on the order of a percent or a few percent. Weaker automatic associations are more widespread and harder to boundary. None of this is hallucination in the clinical sense, and none of it is 'using more of the brain' in the mythical sense.