So sorry to hear of the disappointment you experienced at the salon with your ombré color service.
If the salon is reputable, they will stand behind their work. If you let them know you are unhappy with the differentiation they should offer to fix it for you.
I would definitely not go with box color until you can consult with the salon. Depending upon how they created your ombré, box color might not blend well. Even worse, you might pick up unwanted colors.
I would contact the salon. I would very politely ask them who to talk to regarding an adjustment you would like to request for a recent color service. If the salon thinks you are unhappy, but willing to reasonably request an adjustment, if they are a good salon, they will bend over backwards to work with you.
The problem comes in when salons and/or stylists think a customer is being unreasonable or difficult, or they wait too long after the service to request an adjustment.
You don't have to work with the salon if you're concerned they can't fix it, but why not at least talk to them about it? You could even ask for a different colorist to consult with you on your adjustment.
At any rate, it never hurts to ask? Right? Good luck to you. Let us know how it works out for you.