If light BackColor then white ForeColor - VB.net

so I'm trying to make it so that text over the BackColor will change depending on how Light/dark the BackColor is, so that you can see the text.

So say I pick a dark color then the text should be white, but if I pick a white color then the text should be dark.

This is what I have done so far but it doesn't seem to work:

Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click  
 Using colorDialog As New ColorDialog
            If colorDialog.ShowDialog() = DialogResult.OK Then
                My.Settings.TextBackground = colorDialog.Color
                UpdateTextBack(sender, e)
            End If
        End Using
End sub
 Private Sub UpdateTextBack(sender As Object, e As EventArgs)
        HTML_Edit.RichTextBox1.BackColor = My.Settings.TextBackground
        Text_Edit.RichTextBox1.BackColor = My.Settings.TextBackground
        Button2.BackColor = My.Settings.TextBackground
        If My.Settings.TextBackground.GetBrightness < 0.5 Then
            Button1.ForeColor = Color.White
        Else
            Button2.ForeColor = Color.Black
        End If
    End Sub