Enamel cookware is a kitchen product you'll keep coming back to, and there are several things you should know about how to clean it to avoid having a heavily stained dish.
If the baking soda trick doesn't work, fill your cookware with one part bleach and three parts water, and let it stand for a night or two to remove heavy discoloration.
You can also use Bar Keeper's Friend — a white, powdery mixture of oxalic acid, abrasives, and detergents. While it's safe for enamel, it is much more caustic than other options.