CameraClearFlags.Depth
Description

Clear only the depth buffer.

This will leave colors from the previous frame or whatever was displayed before. See Also: Camera.clearFlags property, camera component.
	// Clear only the depth buffer

camera.clearFlags = CameraClearFlags.Depth;
using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Example() {
        camera.clearFlags = CameraClearFlags.Depth;
    }
}
import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Example() as void:
		camera.clearFlags = CameraClearFlags.Depth