Change color for Bitmap Image - No change in Hex string

I have a Hex string that's coming from postscript file.  
This Hex string is for the image - for which I need to modify RGB color values.  
After modifying color values, I need to write the image back as Hex string in postscript file.  

Below are the methods that I am using. All these methods are working fine including `UpdateColour(bitmapImage)`. I have confirmed it through debugging.

    public static void ProcessImageColourMapping()
    {
        string imageDataSource = "803fe0503824160d0784426150b864361d0f8844625138a4562d178c466351b8e4763d1f904864523924964d27944a6552b964b65d2f984c665339a4d66d379c4e6753b9e4f67d3fa05068543a25168d47a4526954ba648202";
        string imageDataSourceUpdated = GetUpdatedImage(imageDataSource);
    }

    public static string GetUpdatedImage(string strImageDataSource)
    {
        string imageDataSourceUpdated = "";

        byte[] imageBytes = StringToByteArray(strImageDataSource);
        Bitmap bitmapImage = ByteArrayToBitmap(imageBytes);
        UpdateColour(bitmapImage); //I have confirmed by debugging that color values for pixel are actually updated for bitmapImage
        byte[] imageBytesUpdated = BitmapToByteArray(bitmapImage);
        imageDataSourceUpdated = ByteArrayToString(imageBytes);

        return imageDataSourceUpdated;
    }

    public static byte[] StringToByteArray(String imageHexString)
    {
        int numberOfChars = imageHexString.Length / 2;
        byte[] byteArray = new byte[numberOfChars];
        using (var sr = new StringReader(imageHexString))
        {
            for (int i = 0; i < numberOfChars; i++)
                byteArray[i] = Convert.ToByte(new string(new char[2] { (char)sr.Read(), (char)sr.Read() }), 16);
        }
        return byteArray;
    }

    public static Bitmap ByteArrayToBitmap(byte[] byteArray)
    {
        int width = 2144; //width and height are taken from postscript file for testing a single hex string.
        int height = 3;
        Bitmap bitmapImage = new Bitmap(width, height, PixelFormat.Format32bppPArgb);
        BitmapData bmpData = bitmapImage.LockBits(new Rectangle(0, 0, width, height), ImageLockMode.ReadWrite, PixelFormat.Format32bppPArgb);            
        try
        {                
            Marshal.Copy(byteArray, 0, bmpData.Scan0, byteArray.Length);
        }
        finally
        {
            bitmapImage.UnlockBits(bmpData);                
        }
        return bitmapImage;
    }

    public static byte[] BitmapToByteArray(Bitmap bitmap)
    {
        BitmapData bmpdata = bitmap.LockBits(new Rectangle(0, 0, bitmap.Width, bitmap.Height), ImageLockMode.ReadOnly, bitmap.PixelFormat);
        int numbytes = bmpdata.Stride * bitmap.Height;
        byte[] bytedata = new byte[numbytes];
        try
        {
            Marshal.Copy(bmpdata.Scan0, bytedata, 0, numbytes);            
        }
        finally
        {
            bitmap.UnlockBits(bmpdata);
        }
        return bytedata;
    }

    public static string ByteArrayToString(byte[] byteArray)
    {
        StringBuilder hex = new StringBuilder(byteArray.Length * 2);
        foreach (byte b in byteArray)
        {
            hex.AppendFormat("{0:x2}", b);
        }
        return hex.ToString();
    }


    public static void UpdateColour(Bitmap bitmapImage)
    {
        for (int x = 0; x < bitmapImage.Width; x++)
        {
            for (int y = 0; y < bitmapImage.Height; y++)
            {
                Color color = Color.FromKnownColor(KnownColor.Aqua); //For Testing, change all pixels color to Color.Aqua
                bitmapImage.SetPixel(x, y, color);
            }
        }
    }

**Issue:**  
Even though the color for all image pixels are changed:

    bitmapImage.GetPixel(0,0)    "{Name=50ffc8ff, ARGB=(80, 255, 200, 255)}"    - BEFORE UpdateColor()    
    bitmapImage.GetPixel(0,0)    "{Name=ff00ffff, ARGB=(255, 0, 255, 255)}"    - AFTER UpdateColor()

If I compare the two hex strings:  

`imageDataSource` and `imageDataSourceUpdated`, they are same :-(   
Shouldn't they be different after I have modified underlying bitmap color values?

Can please guide what I am missing here.