Gold or Gold Plated?

I purchased a 9ct gold ring from the fourways branch last month for my mums birthday, she used the ring once and kept it away only to find out that the ring had changed colour and now looked bronze instead of the bright yellow gold when i purchased it. i took it to the store and the lady put it in a liquid to clean it, she told me that we mustnt leave it closed in the box as gold needs air to stay the same colour, i have 13 rings, 3 hand chains and 2 neck chains that is gold and they never changed colour even though they are sealed away for over 4 years. Since Saturday we kept the ring outside of the box and guess what it changed colour again. is this ring even gold? or did i get **********?