For an example of a color manga, the Masakazu Katsura Shrine ay 
http://www.geocities.com/Tokyo/Bay/2505/Shrine.html has a chapter of Serial 
Experiments Lain on it's site for a few days more. Go see it now before it's 
gone. It's gorgeous and frightening at the same time. Also, it has a link to 
my Masakazu Katsura club at http://clubs.yahoo.com/clubs/masakazukatsura
Yes, that's a shameless plug.
It's not the only manga I know of that is in full color. I have Hayao 
Miyazaki's Shuna no Tabi (Shuna's Journey), which was his original idea for 
what would later become Mononoke Hime. It is a very different story, but you 
can see the seeds of Mononoke in it. If you can find the book (I got mine 
from Kinokuniya bookstore in Seattle), the translation is posted at 
http://www.nausicaa.net

Mike