Customer unhappy with grout color.

Hello fellow mason lords. Long story short…the customer was very picky about what they wanted and therefore picked out all of their materials for the fireplace remodel. While being completely satisfied with the build and quality of work, they are now unhappy with the grout color and are trying to get it lightened to more of an off-white. They are aware that given they chose all the materials for the job that any attempt to “fix” the color will cost extra. Replacing the mortar is a non option. We all know that you’d be better off just starting over at that point. So, question is if there is any type of process or product I can utilize to either stain the mortar a lighter color, or a product that could kind of decolorize it to a lighter shade? The first picture is the fireplace in its current state, and the second is an example of the color “fix” they are looking for. Thanks ahead of time for any suggestions.