Is it not possible to change the paragraph style applied to these paragraphs so that you only need to change the text that precedes the colon (instead of the text that follows)? For example, you want a Bold lead-in to text that’s set in Regular/Book. That’s the more common case. But that’s not a complete solution ;-) Just makes it a little less complicated.
If you need to apply a character style to text *after* a colon, the first Nested style has to apply the character style [None] through 1 colon. This means “skip this part”. Then add another Nested style after that one that applies your desired character style to the rest of the paragraph. For example, [italic] through [and then for a stop character add something that doesn’t appear in the text, like a Section Marker, the last item in the dropdown list). ID will apply that character style until the end of the paragraph.
Okay so now the problem is what to do with those paragraphs that lack a colon. In the past the only way I’ve solved this is to manually enter some sort of non-printing,0-space-adding character in the paragraphs that lack a colon.
In this situation where you’re skipping the first bit of text, you’ll need to go through the text and in those paragraphs lacking a colon, replace a space character after the word/s in questions with something like a non-breaking space (Type > Insert White Space). That takes up the same amount of room as a space. In your Nested Style set up, in the first one set to apply “None” up to a colon, you’d click to the right of the colon and enter ~S which is code for Non-breaking space.
So: two steps. Comb through the text and manually apply a marker to paragraphs that need one. And then add that marker to the Nested Style.
To make it less complicated, you could adjust your paragraph style as I described in the beginning. If you did that, you’d just need one Nested Style that applies your “make this part look different” Character Style up to or through a colon. Then go through the text and add a “End Nested Style Here” (Type > Insert Special Character > Other) in those locations … they’d be easy to spot because InDesign will have applied the Character Style to the entire paragraph, since it didn’t find a colon.
Does this help at all?