Readonly richtextbox changing color of keyword C#

I have a rich text box that i am using to display an example of the Hello World program and want the keywords such as 'using' 'namespace' 'class' 'static' 'void' 'string' to display blue.

I have got 'using' to display blue but only when the user starts to type or types 'using' I want the richtextbox to be read only not allowing an user input into it

Here is my code:

    private void rtb_TextChanged(object sender, EventArgs e)
    {
        string find = "using";
        if (rtb.Text.Contains(find))
        {
            var matchString = Regex.Escape(find);
            foreach (Match match in Regex.Matches(rtb.Text, matchString))
            {
                rtb.Select(match.Index, find.Length);
                rtb.SelectionColor = Color.Blue;
                rtb.Select(rtb.TextLength, 0);
                rtb.SelectionColor = rtb.ForeColor;
            };
        }
    }

I am having trouble figuring out how to do it for readonly rtb. How could i edit my code to display the blue text on initialize in a readonly rtb

Thanks for your help