I'll second shmem'e response: explanatory comments (or just clear code layout with well chosen names for vars and subs) are the best way to draw attention to specific details in code that you want others to read and understand.

Puttering around with what amounts to ASCII-art will be tedious, and well beyond the patience, attention span or mental capacity of many who (try to) post code here. It'll get especially unwieldy when someone feels compelled to use fairly long lines of code with highlighted stuff at the end.

Even though I kinda like dhoss's suggestion to make sure that "highlighting directives" are really nothing more than commented lines in the code, I have to ask: well then, why not just say a few words in those extra comment lines, instead of counting out periods and carets?