Get pixels of all shades of particular color in PictureBox Image

public enum WhichLayer
    {
        Red=0,
        Green=1,
        Blue=2
    }

private Bitmap GetSpecificLayer(Bitmap image, WhichLayer layer)
{
    Rectangle rect=new Rectangle(0, 0, image.Width, image.Height);
    BitmapData dataIn = image.LockBits(rect, ImageLockMode.ReadOnly, PixelFormat.Format24bppRgb);

    Bitmap result = new Bitmap(rect.Width,rect.Height,PixelFormat.Format8bppIndexed);
    // Keep the original resolotion
    result.SetResolution(image.HorizontalResolution, image.VerticalResolution);
    // Set Grayscale palette
    SetGrayscalePalette(result);
    BitmapData dataOut = result.LockBits(rect, ImageLockMode.WriteOnly, PixelFormat.Format8bppIndexed);

    int strideIn = dataIn.Stride;
    int strideOut = dataOut.Stride;
    int lineOffsetIn = dataIn.Stride - rect.Width * 3;
    int lineOffsetOut = dataOut.Stride - rect.Width;

    // Get layer index
    int ind=0;
    switch(layer)
    {
        case WhichLayer.Blue: ind=0; break;
        case WhichLayer.Green:ind=1; break;
        case WhichLayer.Red: ind = 2;break;
    }
    unsafe
    {
        byte* pIn = (byte*)(void*)dataIn.Scan0;
        byte* pOut = (byte*)(void*)dataOut.Scan0;

        for (int y = 0; y < image.Height; y++)
        {
            for (int x = 0; x < image.Width; x++)
            {
                pOut[0] = pIn[ind];
                pIn += 3;       // skip RGB24=3; RGB32=4...
                pOut++;
            }
            pIn += lineOffsetIn;
            pOut += lineOffsetOut;
        }
    }
    image.UnlockBits(dataIn);
    result.UnlockBits(dataOut);

    return result;
}


private static void SetGrayscalePalette(Bitmap b)
{
    ColorPalette pal = b.Palette;
    for (int i = 0; i < 256; i++)
        pal.Entries[i] = Color.FromArgb(255, i, i, i);
    b.Palette = pal;
}