| I like to paint in layers from light to dark (start with an almost beige, then work to a light tan, dark tan, then a darker brown). Then you can use a very small amount of wash to go around the edges of the seat (and other places where there's a fair amount of wear) and it will thin down through each preceding layer - you decide how much wear you want.
I also suggest use of aftermarket seats in most cases, since most factory leather seats don't come with the level of detail that would help make your seats look authentic. | 
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