I want to have a different notification tone for one specific contact, but I can't figure it out.
I can change the tone only for All Contacts, but not for only for one contact.
You can define a specific ringtone as detailed below. For message notifications you will need to find a messages app that supports that option.
To define contact-specific ringtones:
contacts app open the
Groups Tab. Create a new group with the phone numbers that belong to the contact (home, work, cellular, etc.). Select
Ringtone and choose the tone you want for this contact.