how to timer count down and then count up

Hi i have the next timer and i need to do some other features with the timer. The main idea is start on 60 count down to 0 when the timer gate 30 change the font color to red and when it get 0 display a message and start increasing the number (countup).

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 
 public class Timmer : MonoBehaviour {
 
     public Text Tempo; 
     public float tiempo = 0.0f;
 
 
     
     // Update is called once per frame
     void Update () {
         tiempo -= Time.deltaTime;
         Tempo.text = "Tiempo:" + " " + tiempo.ToString ("f0");
             
     }
 }