I could not make out what you wanted...
I will put it in another way,
I want to find text "A" in the pragraph and change it to "A+B", how can I make "A" and "B" different color after changing in script?
I'm not clear o whether you want to change both A and B, or just one of them.
Either way, you do it by setting the fillColor. You are going to need a reference to a color or a swatch to do that.
But I don't konw how to changethem seperately.