The leather of a 1990 Mercedes driver's seat was worn from entering and leaving the car. The upper layer was gone, so the less colored underground became visible. At www.lederzentrum.de I found a solution that was promised to be simple and affordable.
First I've cleaned the surface as described on the bottle. After the first application of the coloring the good leather showed a deep black gloss while the damaged areas had picked up almost no color. The leather had to dry before color could be applied again. This procedure had to be repeated several times, and finally the damaged areas had become as black as the surrounding leather. Still the surface was rough, but the leather protector brought at leat a little gloss to the bad surface.
The result wasn't perfect, but during one afternoon the damage could be repaired good enough that someone entering the car for the first time wouldn't notice the worn areas. Consumption was minimal, so there should be enough left to treat all of the vehicle's leather surfaces, still leaving enough to redo the repair again when required. Cleaner, Coloring and Leather Protector can be (re-)ordered separately as needed.