I was happy to get a new stove, and chose the Gray Porcelain stovetop burner grates. But was quickly disgusted because these grates turn BLACK, and nothing would clean them. I swear that boiling a pan of water will start to turn them black(?), so discount the fact that grease/cooking splatters do occur. Scrubbing stupid burner grates should not be a daily chore. Desperate snooping found the answer, and I CANNOT/WILL NOT take credit for the process. But, I can share it with you.
Plain, simple Ammonia!
Put the burner grates in a strong plastic bag, add 1/4 cup ammonia, seal the bag, and let it sit overnite. (Last time, I used a 5 gal bucket with lid.) Next day, take out the grates and wash them with a simple scrubbie pad. Maybe a little work on some tough spots, but the black (and grease) come off nicely, and you'll have nice clean grates again. Doubtful? Do a google search on the subject, and you'll find tons of positive results. Remember, this is for PORCELAIN GRATES, of any color. Pure, rough cast iron will not produce the same results. Nor is it recommended to do this with chrome plated oven racks. Give it a try on your burner grates.