It's the same synthetic material as that on the rest of the field but painted brown.
A couple of days ago I watched a bit of a game between the Mets and Marlins. The announcers pointed out and it was visible on close up video that the warning track is not dirt.
Players are expected to run at full speed into a wall while looking in the opposite direction, all for our amusement. It had always been my understanding that some minimal degree of protection was provided when the player made first contact with dirt, as opposed to grass or artificial turf. That way if the player had any brains he would know based on his stride his maximum and minimum distance to impact. But if the player never touches dirt, there is no warning, which is the purpose of the WARNING track!
Whether the issue is big or small, there is no stone left unturned by Major Baseball League commissioner Allan Huber "Bud" Selig in being 100% incompetent.
But a WARNING track that doesn't warn had never occurred to me. And where is former player Tony Clark, who now leads the players union?