ColorChanger.cs

using UnityEngine;
using System.Collections;
using TMPro;

public class ColorChanger : MonoBehaviour 
{
    
        [SerializeField] Gradient gradientText;
        [SerializeField] float gradientSpeed = .1f;

        private TMP_Text m_TextComponent;
        private float totalTime;

        void Awake()
        {
            m_TextComponent = GetComponent<TMP_Text>();
        }


        void Start()
        {
            StartCoroutine(AnimateVertexColors());
        }

        IEnumerator AnimateVertexColors()
        {
            m_TextComponent.ForceMeshUpdate();

            TMP_TextInfo textInfo = m_TextComponent.textInfo;
            int currentCharacter = 0;

            Color32[] newVertexColors;
            Color32 c0 = m_TextComponent.color;

            while (true)
            {
                int characterCount = textInfo.characterCount;

                if (characterCount == 0)
                {
                    yield return new WaitForSeconds(0.25f);
                    continue;
                }

                int materialIndex = textInfo.characterInfo[currentCharacter].materialReferenceIndex;

                newVertexColors = textInfo.meshInfo[materialIndex].colors32;

                int vertexIndex = textInfo.characterInfo[currentCharacter].vertexIndex;

                if (textInfo.characterInfo[currentCharacter].isVisible)
                {
                    //c0 = new Color32((byte)Random.Range(0, 255), (byte)Random.Range(0, 255), (byte)Random.Range(0, 255), 255);
                    float offset = (currentCharacter / characterCount);
                    c0 = gradientText.Evaluate((totalTime + offset) % 1);
                    totalTime += Time.deltaTime;
                    
                    newVertexColors[vertexIndex + 0] = c0;
                    newVertexColors[vertexIndex + 1] = c0;
                    newVertexColors[vertexIndex + 2] = c0;
                    newVertexColors[vertexIndex + 3] = c0;

                    m_TextComponent.UpdateVertexData(TMP_VertexDataUpdateFlags.Colors32);

                }

                currentCharacter = (currentCharacter + 1) % characterCount;

                yield return new WaitForSeconds(gradientSpeed);
            }
        }
}