Wedding bands loosing colour

Mid Dec 2014 my then fianc? and mom went looking for wedding bands and we went to American Swiss Maponya Mall where the assistants where very nice and helpful. We looked around and i saw this beautiful silver ring that i loved and we bought it and so did my fianc?. Around June/July my mother decided to check if everything was ready as the wedding was in Sept 2015 to her horror the rings had turned black and i called the store, spoke to the manager Lebo who was also very helpful and she said we must bring the rings back so they can be cleaned which we did, 4 days later we where called to come and collect them and everything was fine and we where advised to put them in a place where there is light and out of the boxes which we did, a week before the wedding my mother checked the and this time they had turned gold/brownish and we took them back again collected them the Friday before the wedding, however i have just notice that my ring is turning black again and we paid a lot of money for these bands so i would like to change them and we are being told that we need the receipt to change it which is lost as we did not anticipate for this to happen. Please its only been 3 weeks.