Create effect dark and light when game start in unity - unity3d

Question

I am new in unity, i need to create a effect when my game is started:
When game is being started, game scene is dark
When game has already been started, game scene is light.
Please teach me how i can do it, Thanks so much.

Answer 1

1st you have to add Light on your scene from GameObject Menu in unity. If you need On Off that light just add the Javascript to your light. Lot of ways to do this Google it and easy to find.
var times : float;
function Start () {
light.intensity =0;
}
function Update () {
times += Time.deltaTime; // just I added time.
if (times > 5) // or Here you can use Press key events....
light.intensity =8;
}

Answer 2

Here's what I use. I create a pixel texture and use the GUI system to ensure it'll be drawn before everything in the scene. I actually use a display manager to control the order GUI elements are drawn to make sure the overlay is in front of other gui elements.
using UnityEngine;
using System.Collections;
public class Overlay : MonoBehaviour {
//----------------------------------------------------------------------------------------------------//
[SerializeField]
private Color screenColor = Color.black;
[SerializeField]
private float fadeSpeed = 0.5f;
//----------------------------------------------------------------------------------------------------//
static private Texture2D overlay;
static private bool fadeOverlay;
static private bool fadeOverlayIn;
//----------------------------------------------------------------------------------------------------//
public void Awake() {
//Setup the values for the fading overlay.
overlay = new Texture2D( 1, 1 );
overlay.SetPixel( 0, 0, screenColor );
overlay.Apply();
fadeOverlay = true;
fadeOverlayIn = false;
}
public void Update() {
if ( fadeOverlay ) {
if ( fadeOverlayIn ) {
screenColor.a += Time.deltaTime * fadeSpeed;
if ( screenColor.a >= 1 ) {
fadeOverlay = false;
}
} else {
screenColor.a -= Time.deltaTime * fadeSpeed;
if ( screenColor.a <= 0 ) {
fadeOverlay = false;
}
}
overlay.SetPixel( 0, 0, screenColor );
overlay.Apply();
}
}
public void OnGUI() {
if ( fadeOverlayIn || fadeOverlay ) {
GUI.DrawTexture( Camera.main.pixelRect, overlay );
}
}
static public void Toggle() {
fadeOverlay = true;
fadeOverlayIn = !fadeOverlayIn;
}
//----------------------------------------------------------------------------------------------------//
}