Count Number of Unique Colors in an Image

I think we may check if the color is repeated or not, any how this another alternative but in VB.Net

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