Tin is a metal and therefore it reacts with moist as all other metals, although it reacts less as for example silver or copper. So it will get darker after it got wet and this is normal for this product.
If you want to keep it light and shimmering, you have to dry immidiately after it got wet. You can clean it with water and a mild detergent, but you have to dry it directly after cleaning.
Do not wash in a dishwasher.