While they're associated with the color indigo, the Indigo Tribe focuses on compassion and forgiveness. Their power source is the emotional entity known as "Compassion".
However, there is a common phrase associated with the Indigo Tribe:
"I shall not kill, but I shall not yield."
This phrase embodies the core of their philosophy: To use their powers to protect life and bring peace through understanding and empathy, even towards those who have committed terrible acts.
If you are looking for something more specific, perhaps you could tell me what kind of oath you are looking for?