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 anything
	camera.clearFlags = CameraClearFlags.Nothing;

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Example() {
        camera.clearFlags = CameraClearFlags.Nothing;
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Example() as void:
		camera.clearFlags = CameraClearFlags.Nothing