I am studying for the ACT. In my study book it says: use a colon after a complete sentence before a clarification. It also says: use a semicolon to join two complete, related sentences in the place of a conjunction.
Consider the following two sentences:
A. "Text me your information; I want to stay in touch with you."
B. "Soy is used in Japanese cooking: it adds a unique taste to the dishes."
Evidently, sentence A uses a semicolon, and sentence B uses a colon, but in both A and B the second sentence (the one after the colon/semi-colon) clarifies the first sentence (the one before the colon/semi-colon). In A, the second sentence clarifies why I want you to text me your information. In B, the second sentence clarifies why soy is used in Japanese cooking.
I am not sure whether the ACT will test colon vs. semicolon usage with subtle examples like this one.