Silver fillings can leave stains on teeth. When the tooth-colored fillings were placed, the white material might have been thicker at the edges, so you didn't see the stain. Over time, the excess filling material may have worn off at the edges, so you could see the stain.

Or, there may have been a problem with the bonding agent used, and some of the filling material chipped off. This could allow staining of the filling along the line of contact with the tooth. Drinks such as tea, wine, and coffee, and various foods can create stains that are more easily picked up.

Have your dentist check whether there is a smooth transition between the tooth and the filling. If there is a space, then food particles will start to collect in the areas and could lead to decay. In that case, the fillings would need to be redone.