Colons signal readers to keep reading because related thoughts or a list will follow. In this role, colons differ from periods, semicolons, and even commas, all of which signal a pause or even a full stop.
1. Colons link related thoughts, one of which must be capable of standing alone as a sentence.
Colons emphasize the second thought (unlike semicolons, which emphasize both thoughts equally, and dashes, which emphasize the break in the sentence and can emphasize the first thought).
Colons shift emphasis forward: They tend to make the second thought the most important part of the sentence. When such is the case, the colon indicates that explanation or elaboration follows:
The Franklin Shipyard needed one thing to remain solvent: to win ...