Colons may be used to separate independent clauses that are not separated by a conjunction or any other connecting word or phrase.
Semicolons are normally used, but the colon adds emphasis, especially if the first clause leads into the second clause or has a parallel construction.
The second clause begins with a capital letter.
Correct: Grapes are not squeezed: The pulp is pressed.
Some authorities do not capitalize the word following the colon when the subject or point of view does not change between the first and second clauses.
See also Semicolons with Clauses and Special Capitalizing Cases.