I know this is late (hopefully you have email notifications on :-) ) but this is actually a deliberate choice on Microsoft’s part. I believe this was to avoid accidentally encoding any specific race into the icons, although obviously even the faceless variety still have a colour. There used to be a document on their site describing this and lots of other aspects of consistent XP icon design (something like “XP icon design guidelines”), right down the precise perspective you should use. This now seems to have been replaced by the Vista version.

Ah, the original article is still up, here’s the link. Presumably by now you have hired a designer who’s knowledgable enough to have seen this document anyway.

http://msdn2.microsoft.com/en-us/library/ms997636.aspx