give the textfield a name, if it hasn't already (sounds like you put it in a movieclip).
to change the text set the text property of your text field
myTextField_txt.text = "here is the text";
or if it is inside a movieclip
myClip_mc.myTextField_txt.text = "here is the text";
To change the color you would need to put a TextFormat on it. To keep the same fontsize, font etc. you could first get the TextFormat from your existing Textfield, change the color of the TextFormat and set it on the TextField.
// get the Textformat of the first character
var TF:TextFormat = myTextField.getTextFormat(0,1)
// set the font color
TF.color = 0xff0000;
// put it on the whole text
Dealing with the TextArea component, I was use to something like myTextArea.textField.text="Hello"...but I remembered to get to the library, I had to do as you suggested, mcStageText.txtLibraryText.text="Hello" worked great.
I didn't use your Format way, I was able to do this: mcStageText.txtLibraryText.textColor=0xFFFFFF; So far, works just like I wanted.