A Very British Dude * explains proper mustard:

“Listless and radioactive yellow, [American mustard] does not add to the celebration of all that is good and holy in your mouth. The “Actions on” ingesting Proper Mustard are as follows. Your sinuses should clear instantly; your brain should be invigorated and you should develop a nose bleed. English Mustard delivers and it is thus the correct Mustard.”

* I am charitably assuming this is a deliberate oxymoron. The alternative is too horrifying to contemplate.