FAQ: Will leather change color over time?
For un-dyed leathers, the darkness of the leather is generally due to the amount of natural oils found in the leather fibers. If you add oil, the leather will darken. As the leather dries out or if you stress the leather by bending, flexing and stretching it, the oils will escape and the leather will lighten. In general, however, the trend is for leathers to become darker over time.
As with your own skin, exposure to the sun's UV rays will also cause a darkening of the leather.