All tests done with src/emacs -Q. The different background colours come from default- and initial-frame-alist settings done in *scratch* buffer. Looking with an electronic magnifier glass I can see that the glyphs of the directory and sym-link names are anti-aliased (as are the italic names of the text properties items in *Help* buffer, or the incredibly RED group Write permission), while the other glyphs seem to come from a bitmapped font: every pixel has the same intensity level.

BTW, de-composed characters in Mac OS X HFS+ file names are shown as their basic Latin glyphs without further "decoration."