gnus-cite-face-list

gnus-cite-face-list is a variable defined in `gnus-cite.el'.
Its value is

(gnus-cite-1 gnus-cite-2 gnus-cite-3 gnus-cite-4 gnus-cite-5 gnus-cite-6 gnus-cite-7 gnus-cite-8 gnus-cite-9 gnus-cite-10 gnus-cite-11)

Documentation:
*List of faces used for highlighting citations.

When there are citations from multiple articles in the same message,
Gnus will try to give each citation from each article its own face.
This should make it easier to see who wrote what.

You can customize this variable.