you have to eat it right away like all tartares, also have always seen tuna tartare presented with oil as above

Thanks guys! I was watching the tail end of Top Chef and I just saw the girl get kicked of because of her gray tuna tartare, and I was curios to find out to how to prevent it.

Yes indeed, this did happen on Top Chef and for a ball park food stand no less. But the real problem was that the contestant made the tartare a day ahead. There's no way you are going to keep that from going gray...

Yes, when it comes into contact with air, it oxidizes - turning it gray. When I make an Asian-ish tartare, I mix in a bit of sesame oil, otherwise a small amount of olive oil. (I don't bathe it in oil, just add a modest amount and it helps somewhat. The best thing, of course, is to simply to prepare it as close to serving as possible.) Btw, I was recently watching Top Chef and one of the contestants had this problem with a tartare - and Eric Ripert himself commented that she could have avoided it, if she'd mixed in a bit of oil...and when Eric talks fish, I listen. ;-)

It's the tuna coming into contact with the air that causes it. You would need to completely cover it in oil so no air got to it. I've personally never tried this, but that is the reason some store bought pesto has oil on top. You'll notice that any pesto that gets above the oil will turn brown after a day or two. Anyway, I'm not sure how the tuna tartar would be after an oil bath, I'm sure it would still be tasty though.

I've never actually tried this myself, but I've heard adding a little oil can help delay the process.