Question about defining classes

[Chip Seraphine]

This seems like a pretty basic syntactical question, but I somehow 
managed to need help with it anyway :-).  If this exact issue is covered 
in some documentation somewhere, please direct me to it and I will 
happily go RTFM.

Anyway.... I am trying to define  various classes for the machines in my 
environment that describe their characteristics (e.g. use_cups, 
use_xinetd, have_scsi_devices, etc).  I am ending up with a lot of 
situations like this:

groups:
   site1hosts = ( hewey dewey louie screwey )
   site2hosts = ( foo bar qux quux )
   allhosts = ( site1hosts site2hosts )

   hosts_colored_blue = ( dewey )
   hosts_colored_red = ( site2hosts )  #all boxes in site2 is painted red

So far, so good.  The problem comes when I want to set a group class as 
being everybody in group X except for Y and Z:

   #I know this is bad syntax, I am just trying to demonstrate what I 
want to do
   hosts_colored_beige = ( allhosts - ( hosts_colored_red + 
hosts_colored_blue ) )

What is the best way to do something like this?  I know that I could 
define hosts_not_colored_beige as being the union of hosts_colored_red 
and blue, but that doesn't solve my problem.

AtDhVaAnNkCsE,

-Chip