The color of the "_OutlineColor" property of the material. Changing outlineColor will result in an instance of the material.
using UnityEngine; using System.Collections; using TMPro; public class ExampleClass : MonoBehaviour { void Awake() { TextMeshPro textmeshPro = GetComponent<TextMeshPro>(); textmeshPro.outlineColor = new Color32(255, 128, 0, 255); } }