It can be done with a little creativity, but not in a single textbox. Build your text one letter at a time with seperate objects.
https://www.dropbox.com/s/211xd1ir76hq8 ... .capx?dl=0
In the example I used a text object, but you should definitely use spritefont. Also, you can use a loop instead of every x seconds to get rid of the typewriter effect.
To simplify things, you can have two spritefont objects - one red version one black, and just create the suitable one with conditions. You can trigger the color change and end with an escape character in your string that doesn't actually get displayed.
Edit: Alternatively, you may want to look into positioning a tint layer effect over the words you want colored...