Colui (in all its forms) is mostly used with the relative pronoun che (that), functioning both as a subject and as a complement of a verb.

 

Examples:

 

- Per coloro che lo desiderano, il professore Agostini riceve il lunedì fra le 15 e le 18 / For those interested, Professor Agostini's office hours are on Monday between 3 and 6 pm.

- Hai già incontrato colui che prenderà il tuo posto qui? / Have you already met the one who will replace you here?

- Colui che ha visto tutto, si affretti  a parlare / The one who saw everything, please, speak

 

When used stand-alone, without che, colui assumes a pejorative connotation, suggesting detachment and estrangement of the speaker from the person / persons to whom the pronoun refers to. (But mind that this use is quite rare in everyday language). For example:

 

- Non illuderti, colui non parlerà / Don't fool yourself, that one won't talk

 

In old literary texts you can find colui (and coloro) before a noun to indicate possession or authorship. For example:

 

- Per lo colui consiglio (Boccaccio) / Thanks to that person's piece of advice