enriched-mode won't work since it actually saves the file in a different format—one that would prevent
latex from working. Try M-xfind-file-literallyRETpath/to/enriched/mode/fileRET and you will see something like
Content-Type: text/enriched Text-Width: 70
<x-color><param>royalblue4</param>"Return the start and end of the
region as a list, smallest first. If the region is empty or not
active, then bob and eob are used."</x-color>
If you want things highlighted systematically it can probably be done with
font-lock-add-keywords, but if it's entirely arbitrary you would have to write a system that saves another file containing markers and then reads them in on file open and performs the highlighting. It's not impossible but as far as I know, noone has done it yet. See this stackoverflow question.
The question I think is really, what are you trying to accomplish. If it's drawing attention to an area that needs help for example then maybe adding a comment
% TODO: description together with something like fic-mode (which I have never used) would be the best option. With a little more work you could even have it highlight the entire next sentence or something.