They definitely blew it - either grabbed the wrong grout bag, or failed to mix the color powder with the dry grout in advance in far greater quantity than would be needed for the entire job to get consistency, or used liquid colorant and eyeballed the quantity rather than specifically measuring it and the grout powder quantities out precisely - or failed to thoroughly mix it so the bottom of the mix bucket was either white or black, in respective cases. Ideally, they would have convinced you to use a factory pre-mixed color and then remixed the bag themselves before touching it - or maybe they used precolored and failed to mix the container so some of the colorant had separated out in shipping, causing two different shades. Regardless - their mistake and their issue to fix, at no cost to you.
The will undoubtedly try to convince you to go with the darker shade on the current light sections, and just "wash" the surface with darker grout - which can make it match, at least till you get a chipout in the grout or wear through the "wash" with cleaning. There are also grout stains that could darken the lighter grout (if a non-absorbent tile like porcelain), also for a few hours work. Neither colors the grout at depth. Otherwise, short of taking the tile off (typically breaking a good bit of it in the process), the only solution is grinding out all the grout lines one at a time (so tile is supported on three sides while working on fourth) with a grout grabber or die grinder/dremel tool, etc, and then regrouting it. For this to work they have to go in close to at least 1/4" deep (preferably full tile depth) to get good bonding, and to avoid showing a "slip line" of the old grout at the tile edges has to come a bit higher in the grout joint finish surface, so the grout might have to be a bit less recessed than normal. Unless they take the tile out and replace it completely on the dark walls, I would also require they give you an extended warranty for free repair of any tiles that come loose or have grout popouts on the sections they redo - say for 3 years if their normal warranty is 1 year, for instance. That should give most parts that failed to stick time to pop out, though a grinding and regrouting job will never be quite as good as the original because it is basically a repair.
