As soon as we sprayed fine fabric shampoo on it and then we sprayed other rugs the same way on the floor they did not have any color change this rug is the only one that did this all the other rugs cleaned fine it only the back no problems with the front.
I've seen many backings of different colors and all of the darker ones tend to bleed off color during cleaning. Some tufted rugs will show stenciling through the backing, but never wholesale color change like this.
Without cutting it open to see if there's anything black inside, I'd question whether the backing was just completely coated with powdered adhesive that washed away during the cleaning process.