There's conflicting information about that. I heard a while ago that it was possible to get a Shiny one, but then someone said that it wasn't possible after all. This has been going on for a while now, and I think it's best to just assume that all Shiny Celebi are hacked. Unless someone brings up concrete proof that they exist, I won't believe that Celebi can be Shiny.
Actually it is possible to get a shiny celebi. You know the bonus disc for pokemon colleseum that lets you get jirachi? in japan, that bonus disc let you get celebi. and since you get an infinite amount of them, you now have the possibility of getting a shiny one.
...or at least, I think so. I'm not entirely sure if it can generate shiny's...
someone look it up, I'm too lazy