Don't clear anything.
This will leave colors and depth buffer from the previous frame or whatever was displayed before. See Also: Camera.clearFlags property, camera component.
// Don't clear anythingcamera.clearFlags = CameraClearFlags.Nothing;
using UnityEngine;using System.Collections;public class example : MonoBehaviour { void Example() { camera.clearFlags = CameraClearFlags.Nothing; }}
import UnityEngineimport System.Collectionsclass example(MonoBehaviour): def Example(): camera.clearFlags = CameraClearFlags.Nothing