Hi Julia, our No-Knead Crusty White Bread recipe allows for the dough be stored in the refrigerator for up to a week, and we wouldn't recommend going longer than that. Depending on how much yeast is in the recipe, and how warm the dough is going into the fridge, you may find that the dough doesn't rise quite as well towards the end of the week and may also begin to lose structure. Usually you'll get the best rise within the first 2-3 days. As the structure of the dough begins to break down it releases water, and the gray color you noticed was most likely due to oxidation. Neither makes the dough unsafe, but you'll probably have difficulty working with the dough, and it may not rise well or taste as good as it does earlier in the process. 

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