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I'm currently learning the Ruy Lopez opening and typical defences. But every single example I've seen, it is played as white.
Is there any reason why you cannot or shouldn't play Ruy Lopez as black, moving the bishop to b4?
If so, what initial moves by white would preclude or make opening with Ruy Lopez inadvisable when playing as black?
I think that this question comes from a fundamental misunderstanding that many beginners have but more experienced players don't even notice, which I wrote about in this answer.
The short version is that openings describe moves made by both players, not by one player. The Ruy Lopez is 1.e4 e5 2.Nf3 Nc6 3.Bb5, not "1.e4, then 2.Nf3, then 3.Bb5".
Given this, there is no real way to "play the Ruy Lopez as Black", since you're a move behind. The only theoretical way you could get to that position is something like 1.e3 e5 2.e4 Nf6 3.Nc3 Bb4, which would never happen.
...Bb4 mostly makes sense when 1) there is a knight on c3 and you want to undermine its support of e4 and d5, or 2) by checking you can force White to interpose a piece or pawn in an awkward way. These situations do come up but none of them are a "Ruy Lopez as Black".
The opening known as Ruy Lopez is characterized by white playing 3.Bb5 after 1. e4 e5 2.Nf3 Nc6. There are lines where black plays Bb4, but these aren't called Ruy Lopez.
For Bb4/Bb5 to make much sense, there has to be a Knight on c3/c6. Consequently one possibility of Bb4 for black occurs in the Four-Knights-Game, where this is actually the mainline and considered to be equalizing for black:
[FEN ""]
1.e4 e5 2.Nf3 Nc6 3.Nc3 Nf6 4.Bb5 Bb4
