"Iron transfer" onto grout was mentioned as a result, if not prepping ceramic tile properly, before grouting. I think this may be the problem we are facing. Our handyman has done many good jobs, but said he had no idea why the off-white color grout continues to turn rust-colored as soon as it dries, and it is only on one area of the shower- the side wall under a window. We've had it looked at, and no one knows why it keeps turning color. The tiler has scraped the grout out 2 times since, with no change! We are so frustrated, and live with an embarrassing, discolored shower stall! Do you have any ideas, other than replacing the whole thing, and starting again? Please give us some hope! Thanks for your reply.