RichTextBox Text einfärben

private void InsertColoredText(string txt, Color col)
{
    int pos = rtb.TextLength;
    string s = txt + Environment.NewLine;

    rtbTestStatus.AppendText(s);
    rtbTestStatus.Select(pos, s.Length);
    rtbTestStatus.SelectionColor = col;
    rtbTestStatus.DeselectAll();            
}