I think it may have something to do with the whole you know, 200-800 years between the game. It doesn't mean its the same exact Mirror Shield and I doubt it is.
If they are the same, I'd bet it just got rusty or someone painted it. The design isn't the same, though.
I doubt that they are the same shield. If they are, then they must have been repainted, I guess. Seriously, this isn't a deep question worth posting a thread about, all it takes is a little brainwork and the acceptance that not every little detail about everything is put in a Zelda game.
TWW's shield is a Hylian creation, OoT's shield is a Gerudo creation.
Exactly. The mirror shield in The Wind Waker is no less different to the one in Ocarina of Time than the one in Majora's Mask. They're three totally different shields made by totally different people.