The web browsers are just for your account on B&N once it is downloaded than you can side-load it to your nook color. But why buy a nook color and ebooks than read it on your Mac?
I did a search and yes the nook of mac is Intel mac only but all is not lost
Here is calibre it is a nice ebook manager and reader
and it is for your Mac 10.4 OSX get the 0.728.dmg
http://calibre-ebook.com/download_osx
I use this mainly for managing my ebook library.
Dave