Adjust color based on first character of line?

Is there a way of modifying a theme so that the first character of a line modifies its formatting, e.g. text color?

I know I can do the same thing with a tag, so e.g. @person makes the text on the line blue. But would there be a way of making lines blue if the first character is, say, the % symbol?


Late reply, but no I don’t think this is possible.