Need help making Rainbow effect code not dependent on frames

Hello I have a script that changes the color of an image over time. but i have to use the fixed update function for it to work properly i tried using a while loop but kept crashing the program when i used it. i would like to get the script to not be reliant on the fixed update function and would like to use a speed variable to affect the speed of the color transition for the rainbow effect but i am unsure how. here is the script below

 public class ColorText : MonoBehaviour
 {
     [SerializeField]
     private TextMeshProUGUI test;
     public GameObject Panel;
     public Image image;
 
     public bool loop = false;
     public int loopIndex = 0;
     public Color[] characterColors =
     {
         //standard colors
     new Color(0,0,0,0), //clear
     new Color(0,0,0,1), //black
     new Color(0,0,1,1), //blue
     new Color(0,1,1,1), //cyan
     new Color(0.5f,0.5f,0.5f,1), //gray
     new Color(0,1,0,1), //green
     new Color(1,0,1,1), //magenta
     new Color(1,0,0,1), //red
     new Color(1,1,1,1), //white
     new Color(1,0.92f,0.016f,1), //yellow
 
         //custom colors
         //new Color(0,0,0,1), //color name
     };
 
     // Start is called before the first frame update
     void Start()
     {
         image = GameObject.Find("Root").GetComponent<Image>();
     }
 
     // Update is called once per frame
     void FixedUpdate()
     {
         Color();
     }
 
     private void Update()
     {
         if (Input.GetButtonDown("DownArrow"))
             loop = true;
         if (Input.GetButtonDown("UpArrow"))
             loop = false;
     }
 
     void Color()
     {
         image.color = characterColors[loopIndex];
 
         if(loopIndex >= characterColors.Length - 1 && loop)
         {
             loopIndex = 1;
         }
 
         if (loopIndex < characterColors.Length && loop)
         {
             loopIndex++;
         }
         
     }
 
 
 }