I dont see how you are getting those areas dirty to begin with. I mean, I have red interior, and the only thing I think could get dirty is the center storage armrest thing. Everything else seems to be black. Every mustang I've seen has had black interior with a combination of another color. Driverside left armrest should be black to begin with. Perhaps the tan area of the door your hitting with is your knee? I dont know. I always wear pants in mine, even in the summer. My steering wheel has worn a cover since 98. No reason, steering wheel was always nice looking. I just learned to drive with that cover on, it was just easier to swap that when swaping cars. Plus I thought a black cover looked better than the red stock look. Obviously these preventions don't help you now, but perhaps you may consider them in the future.
I would suggest getting an old toothbrush, and soap, and scrubing each area, in a circle motion. If the problem still exists, I suggest what others suggested, painting. You cant brush on paint. It will look horrible. Trust me. It has to be sprayed on. I have mixed and mixed and never could find anything that matched. But then I found a buck fifty can at walmart that was an EXACT match. You may not be as lucky as me, so you may be able to find a place that sells specialty paint. After all, body shops hafta repaint these pieces after airbags go off. If you give up, you can always go with a different color. Just know what will wear off! steering wheel, and center console armrest! They are made with a different material, not to mention they get the most wear and tear from you.
And for the record, I think black on black is boring. Tan and black, though also not my favorite is still better in my opinion.