My kittens were exposed to furnace soot, now their fur is gray. I can’t seem to get them…

Great question. You could use any kitten shampoo but if the soot was greasy, my favorite product is Dawn dishwashing soap. It is mild on their coats and cuts through the grease. Just avoid the eyes, rinse thoroughly and dry them well afterwards. The soot could be an irritant if they've been grooming it off, so if you notice any changes in their behavior, eating or any vomiting, I'd consult your veterinarian.