What Hellion and Neil said—though I think the colons in your examples are absolutely fine and no worse than semicolons. I do agree with Hellion that the ones in your question itself might be better off replaced with a semicolon and a question mark, respectively.
Using a colon when the second clause is clearly a clarification etc. of the first is widely accepted. The fundamental problem is that it could be said that any sentence that follows another is quite often the continuation of a thought initiated earlier, and often provides further information connected with the sentence before it. Consider, as a random example, the second sentence of Neil's answer: it gives further (limiting) information about the statement made in the first sentence. Would it be worthy of a colon? I think it would not; but there is no definite line to be drawn between common and colon-deserving clarification.
If the first sentence seems evidently incomplete without the second one, if it seems to be begging for something, this might indicate that a colon would improve it. That would be the case if the reader, had there been a full stop, would have wondered, "huh, what is this supposed to mean?". A colon notifies the reader that he should not worry if he doesn't fully understand what he has just read: if he will just read on, all will be clear.
For your examples, it also matters what came before each sentence: could the reader have known what was meant by the first clause based on the preceding text or other context, or did he really need the second clause to stop wondering?
This is all rather abstract; I think everyone should decide for himself when circumstances are forcing enough for a colon. Too many colons can be tiring for the reader, because he is pressed on too often, allowed too few pauses to reflect. You may observe my natural usage of colons above, and see how you would use them differently.
In addition, a colon can be used between two clauses in an opposition unmarked by words like but etc.
The contingents of the two countries
had trouble cooperating, owing to
their divergent views on the art of
war. The Germans are fond of order:
the French prefer bravado.
You could say that the French clause explains why the Germans clause is relevant and is hence nothing special; but I think regarding it as a separate use of the colon is somewhat more intuitive.