Floor tile grout is dark


Floor in kitchen/eating area is unsealed grey color for the most part. Around two tiles the grout is very dark, almost black. Not moisture as the color hasn't changed for several days. We can't recall spilling anything in that area. Considering acetone bath to treat it. Suggestions?
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Go to a pool supply store and buy a gallon of chlorine. Carefully, using a Q-tip, dab a bit on one small area and wait a bit and see what happens.
Pool chlorine is nasty stuff. Wear gloves and eye protection and be very careful.
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