mh-highlight-citation-style

mh-highlight-citation-style is a variable defined in `mh-e.el'.
Its value is

gnus

Documentation:
Style for highlighting citations.

If the sender of the message has cited other messages in his
message, then MH-E will highlight these citations to emphasize
the sender's actual response. This option can be customized to
change the highlighting style. The "Multicolor" method uses a
different color for each indentation while the "Monochrome"
method highlights all citations in red. To disable highlighting
of citations entirely, choose "None".

You can customize this variable.

This variable was introduced, or its default value was changed, in version 8.0 of the MH-E package that is part of Emacs 22.1.