Just a quick word from the technical writer:
The en-dasy is used bascially to mean "to" for duration or qauntities. I can't make them on my laptop, but things like April 7-9 would have the en-dash. So would something like "Somewhere about 2.5-2.7 million"
The em-dash is used as we're talking about here. It has a variety of uses, and it does tend you make your writing less formal. I overuse them, I'm afraid.
However, I do agree with Mr. M -- the dash is preferable to the colon in your original.