Each of the Todo modes uses faces to distinguish various aspects of
the display, both structural and informational. For example, the
faces for the date and time strings of todo item headers
(todo-date and todo-time, respectively) by default
inherit the attributes of the corresponding faces used by the Emacs
diary; but when the date and time of a Todo diary item (i.e., an item
lacking todo-nondiary-marker) is earlier than the current date
and time, they are displayed in a different face
(todo-diary-expired). In this way, you can readily recognize
diary items that have “expired” and act accordingly (e.g., by
tagging them as done or by updating the deadlines).
Another example of an informational face is the face used to
distinguish top priority items (todo-top-priority). A third
case is the face used in Todo Categories mode to mark rows of the
table containing categories with only archived items
(todo-archived-only).
The todo-faces customization group contains a complete list of
Todo mode faces and brief descriptions of their use.