I often get compliments on the ways that I paint worn brown leather. It is actually a fairly easy process.
First, you need some way of getting a dark and fairly neutral brown base coat or shading coat. Normally, I either paint a neutral brown color on, then shade it with a brown ink, or just start out with a very dark brown. If you start out with a neutral brown with ink, then you should apply your highlight layers with a little water in them to help the lower colors show through more. If you started out with the dark brown, you should mix the dark brown into your highlight layers in order to create a smooth transition.
The next step is to highlight using a very red brown (like P3 Bloodstone or Citadel Vermin Brown). This is going to be a very prominent color, so go ahead and cover up most of the layers underneath (you just want those layers for shading).
You will now have a very red looking brown color. The next step is the trick to making your leathers look worn. Highlight using a very yellowy brown color (like Citadel’s Snakebite Leather or P3 Rucksack Tan). Keep your blending good as you highlight up and you’ll be able to maintain the illusion well.
In the end, you’ll have a brown color that is more red in the shadows and more yellow in the highlights. This gives the impression that the leather has been out in the sun and is worn.
As always, I have some finished examples:
You can also experiment with these kinds of colors on non-leather surfaces, like the brown scales on this dragon: