From Mechanics in Guide part of ABC of Plain Words by Sir E Gowers (1951)

Opinions vary about the proper uses of the colon in punctuation.

As one of a series of stops ranging in strength from the comma to the full-stop, it has gone out of fashion with the "period" style of writing. Its main functions are now:

  1. Between two sentences in antithesis: Man proposes: God disposes.
  2. To precede an explanation or particularisation, in the same way that the dash may be used; in the words of Fowler "to deliver the goods that have been invoiced in the preceding words":
    News reaches a national paper from two sources: the news agencies and its own correspondents.

    For the second purpose the dash is the colon's weaker relative. See DASH.