The mustard is used to form a type of "glue" for the spices and to help them dissolve into the meat. However, most of the mustard flavor cooks off. Because of this, I just use water. Water works just as well as mustard to dissolve and bind the spices to the meat. Then for the mustard flavor, I simply add dried mustard to my spice rub. However, if you want to use yellow mustard instead - feel free!