I upgraded to a Nook HD+ and no longer need the books on my Nook Color. If I delete them on my Nook Color will that also delete them from my HD+? If I Archive them on the Nook Color and sell it, will those books be accessible to anyone who purchases it?
Do not delete or archive the books.
This action actually takes place on your B&N library online and you will be deleting or archiving them on all devices.
Instead UNREGISTER your Nook Color.
This will remove all B&N content from the device and it will no longer be able to access your online account.
After doing that you might want to do a factory reset as well to ensure that none of your personal information in on the Nook.