For a typewriter effect, you can use a FunctionTimer to display character per character > append to a string > display in textbox.
To work with RichText, you will need to do some string manipulation by detecting an open < and end tag /> and just append them at one go to the string. for a word it fine but for a whole phrase inside a tag it will lose that typewriter effect.
In a dialogue usually we usually emphasis one single word anyway.