Here is some code translated from C# from a school project I did. Not sure if it will help. Also what is Drawing.ColorTranslator.FromOle??? Never seen nor used it before...
Dim length As Integer = richTextBox.TextLength ' at end of text
richTextBox.AppendText(mystring)
richTextBox.SelectionStart = length
richTextBox.SelectionLength = mystring.Length
richTextBox.SelectionColor = Color.Red
Though I have no idea if it will work in vb.net as it was meant to be C#...