I just had my small bathroom remodeled in June. Today my son noticed that my very light gray grout is turning yellow. I took a paper towel with dish detergent and wiped it, most came off. I don't understand why it's changing. We don't smoke in it but it is right off the kitchen. I don't fry much food to say it's grease plus I keep the door closed. I do use a scent plug in but I never see fumes. Can anyone help me figure this out. Thanks