None of these four colors should be considered rare. You can get each of the colors easily if you breed for them.

Here are some great articles on color from some of the message boards, these are all past discussions.
http://www.neapolitan.org/phpBB/viewtopic.php?t=3689&highlight=bbdd

http://www.neapolitan.org/phpBB/viewtopic.php?t=6926&highlight=bbdd

http://www.neapolitan.org/phpBB/viewtopic.php?t=6842&highlight=bbdd

http://www.neapolitan.org/phpBB/viewtopic.php?t=5045&highlight=bbdd

http://neapolitanworld.com/community/modules.php?name=Forums&file=viewtopic&t=419&postdays=0&postorder=asc&start=30