The Colon [:]
A colon is generally a punctuation mark of introduction, signaling 'look ahead', rather than of separating or stopping things. It is used:
a. To introduce a list:
I suggest the following for promotion: Enid Brown, Peter Scott and John Reid.
Use a colon (without a following dash), not a semicolon, to introduce lists.
b. To introduce direct speech:
He said: "I don't give a damn".
A comma could be used instead of the colon.
c. To introduce an explanation, expansion or summary of the first part of a sentence:
There were two problems: his small income and her taste for luxury.
d. Colons are used in proportions and ratios: a 3:1 ratio and in expressing time: 10:25:45 (45 seconds past 10.25 am).
e. There are some occasions when either a colon or a semicolon could be used to join two sentences, but choose a colon if the second one expands, explains or summarises the first one, with the colon signaling 'look ahead!':
Eg.At last he told us Peter's secret: the old tramp had been extremely wealthy, but had gambled his fortune away.