private char LastChar = ' ';
    private void richTextBox1_KeyPress(object sender, KeyPressEventArgs e)
    {
        if (LastChar == '<')
        {
            //Turn On Red after Pressing < character
            richTextBox1.SelectionColor = Color.Red; 
        }
        if (e.KeyChar == '>')
        {
            //Turn Off Red Color
            richTextBox1.SelectionColor = richTextBox1.ForeColor;
        }
        LastChar = e.KeyChar;
    }

    private void button1_Click(object sender, EventArgs e)
    {
        Regex regex = new Regex(@"(<\b[^>]*>)|(</\b[^>]*>)", RegexOptions.IgnoreCase | RegexOptions.Compiled);
 
        for (Match match = regex.Match(richTextBox1.Text ); match.Success; match = match.NextMatch())
        {
            richTextBox1.Select ( match.Index +1, match.Value.Length -2);
            richTextBox1.SelectionColor = Color.Red ;
            richTextBox1.DeselectAll ();
        }
    }