the TextField is stretching because it is INSIDE the box movieclip. The instance of that movieclip is being stretched (when you set _width) so all child objects will also get stretched (unless positioned in the center of the 9-slice guides ).
To get around this, the textField should not be a child object of the 'box'. it should be added to the right hand side. You can add both to a 'container' movieclip if necessary.
As for the colour - why not create a number of symbols in the library e.g. BoxRed, BoxBlue, BoxGreen. Then add them dynamically depending on data from the XML.
The only other way to have full conrol of the colour is to use the drawing API to draw the boxes from scratch with code. This is much easier to do in Actionscipt 3.