The point about emphasis is correct (and perhaps the word amplify or extend might help make the distinction). The Chicago Manual of Style (16th ed.) provides a nice discussion on the two punctuation marks. I started to paraphrase it but the manual has done such a nice job explaining this, I think it is best to show you their exact explanation here.
Semicolon (CMOS 6.54)
"In regular prose, a semicolon is most commonly used between two independent clauses not joined by a conjunction to signal a closer connection between them than a period would."
Colon (CMOS 6.59)
"A colon introduces an element or a series of elements illustrating or amplifying what has preceded the colon. Between independent clauses it functions much like a semicolon, and in some cases either mark may work as well as the other; use a colon sparingly, however, and only to emphasize that the second clause illustrates or amplifies the first. (The colon should generally convey the sense of “as follows.”)"
The summary would be to use a semicolon if the second part of the sentence is related but not a direct amplification and to use a colon if the second part is more than just related (that is, directly amplifies the first independent clause).
Below are two sentences as a simple example to show the difference between the two punctuation marks.
1. The participants filled out the demographic questionnaire; next, they completed the main survey.
2. The study consisted of two parts: first, the participants filled out the demographic questionnaire, and then they completed the main survey.
In Sentence 1, the second part of the sentence gives information that is related to but does not directly amplify (i.e., provide more detail) the information in the first part of the sentence.
In Sentence 2, the text after the colon does provide more detail about the first part of the sentence. Note that there are more concise ways to say this (e.g., "The study consisted of two measures: a demographic questionnaire and the main survey." But I just used a more roundabout version out of lack of creativity to give an example with two independent clauses.).
Also note that in Chicago style, the first word after the colon is not capitalized, but in APA it would be capitalized if the text following the colon represents a grammatically complete sentence, such as in your second example above (I'm not sure about Oxford style off the top of my head).
Getting back to your second example, I think you could justifiably use either punctuation mark there. As you mentioned, the colon would give slightly more emphasis to the second part of the sentence than a semicolon would. Your question is very reasonable and I think it stems mostly from the fact that your example could go either way. In other cases, such as in my two sentences, it is a bit clearer which punctuation mark is appropriate. In cases when you aren't sure, try to think of the distinction between just related (semicolon) and direct amplification (colon). If you still aren't sure, just break it into two sentences. :)
On a site note, in your second example, I would change "are" to "have been" or "will be" depending on the desired connotation.