How can I make objects change color, then fade/return to its original color?

1) Make two cubes , and check OnTrigger on one of them under the box collider

2) Attach the following script to the cube you want to change the color of ( to red on contact and then fading back to green on exit)

 using UnityEngine;
 using System.Collections;
 
 public class testScript : MonoBehaviour {
 
     bool toGreen,toRed;
     float lerpValue,lerpDuration;
 
     void Start () 
     {
         toGreen=false;
         toRed=false;
         lerpValue=0f;
         lerpDuration=1f;
     }        
 
     void Update()
     {
         if(toRed)
         {
             //change color to red
             gameObject.renderer.material.color = Color.red;
             toRed=false;
         }
         else if(toGreen)
         {
             // fade color to green
             if (lerpValue< 1){ 
 
                 lerpValue += Time.deltaTime/lerpDuration;
                 gameObject.renderer.material.color = Color.Lerp(Color.red, Color.green, lerpValue);
             }
             else
             {
                 toGreen=false;
                 lerpValue=0f;
             }
     
         }
 
     }
 
     void OnTriggerStay()
     {
         toRed=true;
     }
 
     void OnTriggerExit()
     {
         toGreen=true;
     }
 
 
 }

You can change the Lerp Duration variable to change how long it takes to fade.

Theres is also an option of doing this with OnCollision instead of OnTrigger