How to use a pixel shader with spritebatch. Minimal example GreyScale

// game1 DX desktop project.

using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;

namespace HowToPixelShaderWithSpriteBatch
{

    public class Game1 : Game
    {
        GraphicsDeviceManager graphics;
        SpriteBatch spriteBatch;
        Texture2D texture;
        Effect effect;

        float percent = 1.0f;

        public Game1()
        {
            graphics = new GraphicsDeviceManager(this);
            Content.RootDirectory = "Content";
        }
        protected override void Initialize()
        {
            base.Initialize();
        }

        protected override void LoadContent()
        {
            spriteBatch = new SpriteBatch(GraphicsDevice);

            texture = Content.Load < Texture2D >("rabbit");
            effect = Content.Load<Effect>("ShaderForSpriteBatch");
            effect.CurrentTechnique = effect.Techniques["BasicColorDrawing"];
        }

        protected override void UnloadContent()
        {
        }

        protected override void Update(GameTime gameTime)
        {
            if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed || Keyboard.GetState().IsKeyDown(Keys.Escape))
                Exit();

            if (Keyboard.GetState().IsKeyDown(Keys.Space))
            {
                percent -= .01f;
                if (percent <= 0)
                    percent = 1.0f;
            }
            effect.Parameters["percent"].SetValue(percent);

            base.Update(gameTime);
        }

        protected override void Draw(GameTime gameTime)
        {
            GraphicsDevice.Clear(Color.CornflowerBlue);

            spriteBatch.Begin(SpriteSortMode.Immediate, null, null, null, null, effect, null);
            spriteBatch.Draw(texture, new Rectangle(0, 0, 300, 300), Color.White);
            spriteBatch.End();

            base.Draw(gameTime);
        }
    }
}

// a basic pixel shader

#if OPENGL
    #define SV_POSITION POSITION
    #define VS_SHADERMODEL vs_3_0
    #define PS_SHADERMODEL ps_3_0
#else
    #define VS_SHADERMODEL vs_4_0_level_9_1
    #define PS_SHADERMODEL ps_4_0_level_9_1
#endif

float percent;

sampler2D TextureSampler : register(s0)
{
    Texture = (Texture);
};

float4 MainPS(float4 position : SV_Position, float4 color : COLOR0, float2 TextureCoordinates : TEXCOORD0) : COLOR0
{
    float4 col = tex2D(TextureSampler, TextureCoordinates)* color;
    col.rgb = (col.r + col.g + col.b) / 3.0f * percent; // grey scale and darken
    return col;
}

technique BasicColorDrawing
{
    pass P0
    {
        PixelShader = compile PS_SHADERMODEL MainPS();
    }
};