Am I right in saying grout is a very fine cement which makes it water based, so therefore keeping it damp with water stops it curing which makes it easier to remove, application by application.
This is what I say to people who call saying they've spilt emulsion paint, just keep sraying water on it to keep it damp so it doesn't set.
The big problem with this grout job is that it will have a grey dye in it, and now no doubt 1001, vanish etc etc.