Unfortunately, in most cases this is not true and the yellow-brown tones are signs of deterioration. These colors are what we perceive to be the natural color of older photographs because we see so many photographs today that are deteriorating. Sepia toning converts the metallic silver in a photograph to a sulphide which will significantly increase the life of the photograph. As you can see, there are two major benefits from sepia toning. The new photograph is made to look more like those from the mid to late 19th century while it significantly increases the life of the photograph.
Some labs make their photographic copies on color photo paper.
This enables them to use filtration to simulate the sepia effect. However, this creates two problems as this is not a true sepia process and does not increase the life of the the photograph.