Say your text field had an instance name of..."mytxt"

You can change values using AS like this: (I don't think you can do it from the components panel.)

mytxt.background = true; //need to set this b/c the default is false
mytxt.backgroundcolor = ffffff;

There are other values you can change, such as:

border
borderColor
textColor

Along with many others, including alignment and such.

Hope this helps.