I have noticed that comment type sub-headings get highlighted with a different colour only when
"outline-minor-mode-highlight" is set to "override". But then the code headings do not keep the
code highlighting. Then if you put "outline-minor-mode-highlight" to "append", so that that code
retains the code highlighting, the sub-headings levels will use a single highlighting colour.

This outline highlighting implementation in "outline.el" has not been very well thought through.
Code highlighting should be retained, whilst comment type highlighting should use a different colour
according to the heading level.