I used to own a detailing shop. If the mustard is actually dried on the paint, do not try to "Scratch it" off, you may take the paint with it. Bring it to a shop to have it professionaly cleaned. A basic polish and wax on a small car should not cost more than 80-100 depending on where you live. They will use a egg crate wheel that will heat up the paint and pull out what may have soaked into the paint. Make sure to tell them exactly what it is and how long it has been on, as well as what you have used to try to remove it (OTC chems). Most pro shops will offer some type of guarentee, make sure you ask what there policy is on not being able to clean it off, or further damage. Most of the time you can stay and watch. It should come clean, but being black, there is a slight chance that it may fade just slightly. Good luck and take care of it quick.
HT