Your best bet will be to pick up some good, quality fabric dye and mix it a bit light on water so that it's not too runny. Use a small paint brush or an old toothbrush to work the dye into the affected area of the jacket, until the area appears roughly a shade or two darker than the rest of the jacket. (It will dry lighter than it looks when wet.)
Allow the jacket to dry in a cool, well-ventilated area for at least 24 hours.
Also, depending on what you used to remove the diesel marks, this may affect the leather's ability to take the dye.
Hope this helps.
handygirl, November 2008
