I read in a punctuation guide that a dash can be used in place of a colon. They provided the following example:
The white sand, the warm water, the sparkling sun—this is what brought them to Hawaii.
However, just to clarify, but what precedes a colon must be an independent clause. Is that not the case for an em dash?
Rules were made to be broken. In formal writing, your example might be considered questionable because it does not begin with an independent clause. However, you will regularly see sentences like this in published works of fiction, because they allow a more natural, conversational flow.
The same sentence could be written with a colon, but it would have a different "feel" to it. The problem is that dashes are also used to denote interruption, while colons are not. So with a dash, your sentence connotes a stream of consciousness, with the narrator abruptly generalizing from specific elements to Hawaii as a whole. With a colon, it connotes an itemized list summarized by the thought about Hawaii, a much more dry and academic style of writing. But if you want to be dry and academic, you probably want to recast the whole sentence (and move Hawaii to the front), so it doesn't really work with the colon.
