Hi Olivia, you didn't mention what your normal feeding routine is, but blue/gray are not normally worrisome colors, and this is probably mostly due to oxidation. If you're not already feeding your starter twice daily at room temperature, increasing the frequency of feedings for a few days might be all that is necessary to improve the appearance, aroma and rising behavior of your starter. In addition, I would try to feed your starter according to our maintenance routine (equal parts by weight of starter, water and flour) and see if that makes a difference. If your starter is being fed partly with rye flour or whole wheat flour, I've noticed that these types of flours tend to show more discoloration on the surface due to oxidation than a white flour starter usually generates. Again, this isn't anything to worry about. 

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