Use leather dye to add a second layer of color to a garment. You can create a subtle aged appearance, or a mottled/stained appearance. This would obviously not work on a black garment.
You will want to remove any protective coatings from the garment with acetone and sandpaper first. Getting the garment damp helps the dye soak in. Use an artist’s paintbrush or foam brush to paint dye into the crevices of the garment.
Paint can be useful for creating stains or artificial damage. Acrylic or latex paint is best.
Angelus is a paint brand that works especially well with leather and won’t crack like regular acrylic paint. You may want to have some Angelus paint duller on hand to ensure a matte finish (they only offer matte in black and white). Click here for their full color chart.
Oil paints can also work quite well, but require a longer drying time.
Leather soaks up oil and is preserved by it. Try a plant-based oil, like walnut oil, on a lighter-colored garment to create stains or darkened areas.