Count Number of Unique Colors in an Image

Public Shared Function GetImageColorsCount(ByVal bitmap As Bitmap) As Integer
        Dim ColorList As New System.Collections.Generic.List(Of Integer)
        Dim clr As Integer
        For y As Integer = 0 To bitmap.Height - 1
            For x As Integer = 0 To bitmap.Width - 1
                clr = bitmap.GetPixel(x, y).ToArgb()
                If Not ColorList.Contains(clr) Then ColorList.Add(clr)
            Next
        Next
        GetImageColorsCount = ColorList.Count
End Function