Marshal can't compare ?

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VB

Hi , all i try to compare and replace different pixel between 2 bitmap images with white color
but i got Error statement with marshal to compare
Any solution Help me compare and replace it ?
need Help!!!
Best Regard,

Public Function ComparePixel(ByVal input As Bitmap) As Bitmap
    tempref = (System.Drawing.Image.FromFile("C:\refBG\img0.bmp"))
    Dim CloneRect As New RectangleF(0, 0, input.Width, input.Height)
    Dim ClonedBitmap = input.Clone(CloneRect, PixelFormat.DontCare)
    Dim x As Integer
    Dim y As Integer
    Dim c1, c2 As Color
    Dim bmd As BitmapData = ClonedBitmap.LockBits(New Rectangle(0, 0, input.Width, input.Height), System.Drawing.Imaging.ImageLockMode.ReadOnly, ClonedBitmap.PixelFormat)
    Dim refbmd As BitmapData = tempref.LockBits(New Rectangle(0, 0, input.Width, input.Height), System.Drawing.Imaging.ImageLockMode.ReadOnly, tempref.PixelFormat)
    Dim index, refindex As Integer
    For y = 0 To input.Height
        For x = 0 To input.Width
            index = y * bmd.Stride + x * 4
            refindex = y * refbmd.Stride + x * 4
            If (Color.FromArgb(Marshal.ReadByte(bmd.Scan0, index + 2), Marshal.ReadByte(bmd.Scan0, index + 1), Marshal.ReadByte(bmd.Scan0, index)).ToArgb()).Equals(Color.FromArgb(Marshal.ReadByte(refbmd.Scan0, index + 2), Marshal.ReadByte(refbmd.Scan0, index + 1), Marshal.ReadByte(refbmd.Scan0, index))) Then
                Marshal.WriteByte(bmd.Scan0, index + 2, 255)
                Marshal.WriteByte(bmd.Scan0, index + 1, 255)
                Marshal.WriteByte(bmd.Scan0, index, 255)

            End If
        Next
    Next
    ClonedBitmap.UnlockBits(bmd)
    tempref.UnlockBits(refbmd)
    Return ClonedBitmap
End

Function