The write up for the Spinward Marches lists the Cronor Sub-Sector as Amber Zone. I think (don't have my books here) that most of the Core-Spinward sections of the Marches, outside the Imperium were considered Amber Zones, even if the individual worlds were not labled as Amber.
Of course, that begs the question, what about an Amber Zoned world within an Amber Zone Sub-Sector?