Faces for component matches

The portions of a candidate matching each component get highlighted in one of four faces, ‘orderless-match-face-?’ where ‘?’ is a number from 0 to 3. If the pattern has more than four components, the faces get reused cyclically.

If your ‘completion-styles’ (or ‘completion-category-overrides’ for some particular category) has more than one entry, remember than Emacs tries each completion style in turn and uses the first one returning matches. You will only see these particular faces when the ‘orderless’ completion is the one that ends up being used, of course.