A guy I know had his girlfriend make the jacket for his Finn costume. She made the jacket out of imitation leather in a light colour much like your jacket and dabbed on mustard-brown acrylic paint. The dabbing process made it somewhat uneven, making the material looking more like leather than before.
I was told by the seller this is real leather, so I'm hoping I'll be able to darken it without painting it.
I am wondering if anyone has ever tried mink oil on something as large as a jacket.