But as I understand it, if I turn on this option, I will not be able to view the whole pattern during the work, will I? Or is it possible to set the highlight only for a particular unused color?
You can select ANY one color for highlight. If you set it for an unused color, than the entire display will appear very faded. Then you can see your underlay.
Just remember to turn off the highlighting when you get back to active creation or it will be difficult to see the stitches you add.