I cant make a stoplight delay from green to yellow, then to red, when my boolean is off. How can I make it delay and actually get yellow to show?

At the moment the current code, just makes the light Green when I make the bool true. But when I set the bool to false, it returns immediately to red and never attempts yellow. Ive tried corutines and using WaitForSeconds and it does not work out for me. here is what I have so far.

 public class TrafficSystem : MonoBehaviour
 {
 
     [SerializeField]
     bool SwitchLights;
 
     [SerializeField]
     GameObject RedLight;
 
     [SerializeField]
     GameObject YellowLight;
 
     [SerializeField]
     GameObject GreenLight;
 
     // Use this for initialization
     void Start()
     {
         RedLight.SetActive(true);
         YellowLight.SetActive(false);
         GreenLight.SetActive(false);
     }
 
     // Update is called once per frame
     void Update()
     {
         ChangeLights();
         ToggleLights();
     }
 
     void ChangeLights()
     {
         if (SwitchLights == true)
         {
             RedLight.SetActive(false);
             GreenLight.SetActive(true);
         }
 
         if (SwitchLights == false)
         {
             {
                 GreenLight.SetActive(false);
                 RedLight.SetActive(true);
             }
         }
     }
 
     void YellowDelay()
     {
         YellowLight.SetActive(true);
     }
     void ToggleLights()
     {
         if (Input.GetKeyDown(KeyCode.F))
         {
             if (SwitchLights)
             {
                 //turn on the light
                 SwitchLights = !SwitchLights;
             }
             else
             {
                 //turn off the light
                 SwitchLights = !SwitchLights;
             }
         }
     }
 }