Source: http://www.codeproject.com/KB/cs/RTFSyntaxColour.aspx

private void AddColouredText(string strTextToAdd)
{
    //Use the RichTextBox to create the initial RTF code
    txtbox.Clear();
    txtbox.AppendText(strTextToAdd);
    string strRTF = txtbox.Rtf;
    txtbox.Clear();
 
    /* 
     * ADD COLOUR TABLE TO THE HEADER FIRST 
     * */
 
    // Search for colour table info, if it exists (which it shouldn't)
    // remove it and replace with our one
    int iCTableStart = strRTF.IndexOf("colortbl;");
 
    if (iCTableStart != -1) //then colortbl exists
    {
        //find end of colortbl tab by searching
        //forward from the colortbl tab itself
        int iCTableEnd = strRTF.IndexOf('}', iCTableStart);
        strRTF = strRTF.Remove(iCTableStart, iCTableEnd - iCTableStart);
 
        //now insert new colour table at index of old colortbl tag
        strRTF = strRTF.Insert(iCTableStart,
            // CHANGE THIS STRING TO ALTER COLOUR TABLE
            "colortbl ;\\red255\\green0\\blue0;\\red0\\green128\\blue0;\\red0\\green0\\blue255;}");
    }
 
    //colour table doesn't exist yet, so let's make one
    else
    {
        // find index of start of header
        int iRTFLoc = strRTF.IndexOf("\\rtf");
        // get index of where we'll insert the colour table
        // try finding opening bracket of first property of header first                
        int iInsertLoc = strRTF.IndexOf('{', iRTFLoc);
 
        // if there is no property, we'll insert colour table
        // just before the end bracket of the header
        if (iInsertLoc == -1) iInsertLoc = strRTF.IndexOf('}', iRTFLoc) - 1;
 
        // insert the colour table at our chosen location                
        strRTF = strRTF.Insert(iInsertLoc,
            // CHANGE THIS STRING TO ALTER COLOUR TABLE
            "{\\colortbl ;\\red128\\green0\\blue0;\\red0\\green128\\blue0;\\red0\\green0\\blue255;}");
    }
 
    /*
     * NOW PARSE THROUGH RTF DATA, ADDING RTF COLOUR TAGS WHERE WE WANT THEM
     * In our colour table we defined:
     * cf1 = red  
     * cf2 = green
     * cf3 = blue             
     * */
 
    for (int i = 0; i < strRTF.Length; i++)
    {
        if (strRTF[i] == '<')
        {
            //add RTF tags after symbol 
            //Check for comments tags 
            if (strRTF[i + 1] == '!')
                strRTF = strRTF.Insert(i + 4, "\\cf2 ");
            else
                strRTF = strRTF.Insert(i + 1, "\\cf1 ");
            //add RTF before symbol
            strRTF = strRTF.Insert(i, "\\cf3 ");
 
            //skip forward past the characters we've just added
            //to avoid getting trapped in the loop
            i += 6;
        }
        else if (strRTF[i] == '>')
        {
            //add RTF tags after character
            strRTF = strRTF.Insert(i + 1, "\\cf0 ");
            //Check for comments tags
            if (strRTF[i - 1] == '-')
            {
                strRTF = strRTF.Insert(i - 2, "\\cf3 ");
                //skip forward past the 6 characters we've just added
                i += 8;
            }
            else
            {
                strRTF = strRTF.Insert(i, "\\cf3 ");
                //skip forward past the 6 characters we've just added
                i += 6;
            }
        }
    }
    txtbox.Rtf = strRTF;
}