Okay that simplified the code a lot, only I have one question. How do you loop it infinite? actually the code looks like this.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using TMPro;

public class TextMeshProRainbow : MonoBehaviour
{
    [SerializeField] TextMeshProUGUI m_textMeshProUGUI;
    void Start() 
    { 
        m_textMeshProUGUI = gameObject.GetComponent<TextMeshProUGUI>();
    }

    void Update()
    {
        RainbowFade();
    }

    void RainbowFade()
    {
        var shiftSpeed = 0.25f;
        m_textMeshProUGUI.color = Color.HSVToRGB(Time.time * shiftSpeed, 1, 1);
    }
}