ColorOS on the Oppo smartphone allows us to assign a different ringtone for each contact, and in addition to the standard tones, a separate ringtone can be used.

Android 10 (ColorOS 7.0)
Step 1:  Open the  Contacts -App
Step 2:  Tap on  Contact  you want
Step 3:  Open the  Menu
Step 4:  Tap on  Edit
Step 5:  Tap on  Ringtone
Step 6:  Choose desired  Ringtone
Step 7:  Tap on  Back
Step 8:  Confirm with  Save
  1. Open the Contacts-App
  2. Tap on Contact you want
  3. Open the Menu
  4. Tap on Edit
  5. Tap on Ringtone
  6. Choose desired Ringtone
  7. Tap on Back
  8. Confirm with Save

Oppo Instructions