IIRC, there's no missing artwork in the color S2, except for the back cover (posted above by Plaag). It's a shame they didn't include this piece because it's very evocative & features the same characters as several of the other Otus interior pieces. I believe there's no textual differences between the
mono & color S2's (I did compare them once), just a lot of extra artwork in the color version. The
mono lists only Otus, Sutherland and Darlene with artwork credits. The color adds a number of Roslof pieces and some by Dee and DSL and the extremely cool Otus exterior map.
Wizards.com posted a bunch of scans of S2 artwork. They show the cover of the
mono, but there's some artwork that's only from the color version:
http://www.wizards.com/default.asp?x=dnd/ag/20051207a
(the color interior map is not from either)
Other old threads that may be of interest:
Differences in content between mono & color modules -
viewtopic.php?t=4445
White Plume Mountain checklist:
viewtopic.php?t=3097