I don't think it's possible to define text color
in the stringtable. As far as I know the stringtable is mostly for ease of translation into other languages. If you only intend your mission to be in English, I don't think you really need one.
Anyway, there is definitely a way to get colored text in your mission. I haven't done it, but I'm 99% certain that the way is to use custom resources, specifically custom titles. General Barron wrote a tutorial
here.
Good luck.
EDIT: I just realized that custom titles may only allow one color for a given set of titles. Experiment around a little to be sure, but if that's the case, suggestions:
(1) See if there's a way to "stack" the custom titles one on top of the other, so it looks like:
Anderson:
We've got to flank that machine gun
(2) Use a custom object instead of custom text. A custom object might take up more of the field of view, though.
(3) But frankly, if I were you I'd just put the persons name in all caps.
ANDERSON: We've got to flank that machine gun!
I don't think anyone would nitpick.