I've seen this a few times now and it confuses me, especially when my editor does it. According to the Chicago Manual of Style, 17th Edition, a colon can only be used where a period could. In other words, after a complete sentence. So, using it in the following situations should not work:
"My next wish is: to bring my father back."
"Consider this: I am the king of this castle."
"The real question is this: are you a man or a mouse?"
- "My next wish is: to bring my father back."
- "Consider this: I am the king of this castle."
- "The real question is this: are you a man or a mouse?"
In all three examples, the first clause is dependent and therefore could not stand on its own. By that logic, the correct form of each would be as follows:
"My next wish is, to bring my father back."
"Consider this, I am the king of this castle."
"The real question is this, are you a man or a mouse?"
- "My next wish is, to bring my father back."
- "Consider this, I am the king of this castle."
- "The real question is this, are you a man or a mouse?"
Am I over analyzing here?
CMoS also adds that "A colon may also be used to introduce quotation or a direct but unquoted question, especially where the introduction constitutes a grammatically complete sentence."
The key word I'm looking at here is "especially," which would indicate that the case of a "grammatically complete sentence" is not always necessary.