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If you're cooking with acids (tomatoes and wine being the most likely) the acid will react chemically with the cast iron; I'm not sure if this will turn food grey, having been warned against it early in life, but it seems a good possibility. Also acids can turn some foods grey without the cast iron- green beans are the only one that springs to mind, but there are quite a few others.
Is the pot seasoned well with oil? I once left my cast iron pan in the oven waaaay too long trying to season it, all of the oil burned off, and the whole pan turned silver. I salvaged the pan, but the lack of oil/seasoning might impart some grey/silverish color to your food? But, Smaug's answer about using acid in the pan might also be the issue?