残像いっぱい色いっぱい

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■ソース
Game1.cs

namespace WindowsGame
{
    public class Game1 : Microsoft.Xna.Framework.Game
    {
        GraphicsDeviceManager graphics;
        ContentManager content;

        VertexPositionTexture[] vertices =
        {
            new VertexPositionTexture( new Vector3(  0.0f,  0.0f, 0.0f ),new Vector2( 0.0f, 0.0f ) ),
            new VertexPositionTexture( new Vector3( 24.0f,  0.0f, 0.0f ),new Vector2( 1.0f, 0.0f ) ),
            new VertexPositionTexture( new Vector3(  0.0f, 24.0f, 0.0f ),new Vector2( 0.0f, 1.0f ) ),
            new VertexPositionTexture( new Vector3( 24.0f, 24.0f, 0.0f ),new Vector2( 1.0f, 1.0f ) )
        };
        VertexDeclaration vdecl;

        VertexPositionColor[] verticesB =
        {
            new VertexPositionColor( new Vector3(   0.0f,   0.0f, 0.0f ),new Color( 0,0,0,4 ) ),
            new VertexPositionColor( new Vector3( 800.0f,   0.0f, 0.0f ),new Color( 0,0,0,4 ) ),
            new VertexPositionColor( new Vector3(   0.0f, 600.0f, 0.0f ),new Color( 0,0,0,4 ) ),
            new VertexPositionColor( new Vector3( 800.0f, 600.0f, 0.0f ),new Color( 0,0,0,4 ) )
        };
        VertexDeclaration vdeclB;

        Texture2D texture;
        BasicEffect basicEffect;
        const int particleMax = 100;
        Particle[] particle = new Particle[particleMax];

        public Game1()
        {
            graphics = new GraphicsDeviceManager(this);
            content = new ContentManager(Services);
        }

        protected override void Initialize()
        {
            vdecl = new VertexDeclaration(graphics.GraphicsDevice, VertexPositionTexture.VertexElements);
            vdeclB = new VertexDeclaration(graphics.GraphicsDevice, VertexPositionColor.VertexElements);
            basicEffect = new BasicEffect(graphics.GraphicsDevice, null);
            basicEffect.TextureEnabled = true;

            basicEffect.World = Matrix.Identity;
            basicEffect.View = Matrix.Identity;
            //原点（ 0, 0 ）を画面右上に設定
            basicEffect.Projection = Matrix.CreateOrthographicOffCenter(
                0.0f,                            // 左座標
                this.Window.ClientBounds.Width,  // 右座標　Window幅
                this.Window.ClientBounds.Height, // 下座標　Window高さ
                0.0f,　　　　　　　　　　　　　　// 上座標
                0.0f,                            // ニアクリップの距離
                1.0f);                          // ファークリップの距離
            //パーティクルの生成
            Random ran = new Random();
            for (int i = 0; i < particleMax; i++)
                particle[i] = new Particle(
                    (float)ran.Next(700),
                    (float)ran.Next(500),
                    (float)ran.Next(360) * 3.14159f / 180.0f);
            base.Initialize();
        }

        protected override void LoadGraphicsContent(bool loadAllContent)
        {
            if (loadAllContent)
            {
                texture = content.Load<Texture2D>("ball");
            }
        }

        protected override void Update(GameTime gameTime)
        {
            foreach (Particle p in particle)
                p.Move();
            base.Update(gameTime);
        }

        protected override void Draw(GameTime gameTime)
        {
            //---------------------------------------------------------------------
            basicEffect.Begin();
            graphics.GraphicsDevice.RenderState.AlphaBlendEnable = true;
            graphics.GraphicsDevice.RenderState.SourceBlend = Blend.SourceAlpha;
            graphics.GraphicsDevice.RenderState.DestinationBlend = Blend.InverseSourceAlpha;
            basicEffect.VertexColorEnabled = true;
            basicEffect.View = Matrix.CreateTranslation(new Vector3(0,0,0));
            graphics.GraphicsDevice.VertexDeclaration = vdeclB;
            basicEffect.CurrentTechnique.Passes[0].Begin();
            graphics.GraphicsDevice.DrawUserPrimitives<VertexPositionColor>(
                                PrimitiveType.TriangleStrip, verticesB, 0, 2);
            basicEffect.CurrentTechnique.Passes[0].End();
            //---------------------------------------------------------------------
            basicEffect.VertexColorEnabled = false;
            graphics.GraphicsDevice.RenderState.DestinationBlend = Blend.One;
            graphics.GraphicsDevice.RenderState.AlphaDestinationBlend = Blend.One;
            graphics.GraphicsDevice.VertexDeclaration = vdecl;
            foreach (Particle p in particle)
            {
                basicEffect.CurrentTechnique.Passes[0].Begin();
                graphics.GraphicsDevice.Textures[0] = texture;
                basicEffect.View = Matrix.CreateTranslation(p.Pos);
                graphics.GraphicsDevice.DrawUserPrimitives<VertexPositionTexture>(
                    PrimitiveType.TriangleStrip, vertices, 0, 2);
                basicEffect.CurrentTechnique.Passes[0].End();
            }
            basicEffect.End();
            base.Draw(gameTime);
        }
    }
}

Particle.cs

using System;
using System.Collections.Generic;
using System.Text;
using Microsoft.Xna.Framework;


namespace WindowsGame
{
    class Particle
    {
        public Vector3 Pos;
        public Vector3 Direction;

        public Particle(float x, float y, float ang)
        {
            Pos = new Vector3(x, y, 0);
            Direction = new Vector3((float)Math.Sin(ang) * 4.0f, (float)Math.Cos(ang) * 4.0f, 0);
        }

        public void Move()
        {
            Pos += Direction;
            if ((Pos.X < 0) || (Pos.X > 800-32))
                Direction.X = -Direction.X;
            if ((Pos.Y < 0) || (Pos.Y > 600-32))
                Direction.Y = -Direction.Y;
        }
    }

}

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■結果