Pawn promotion irregularity

I'm pretty sure it says in FIDE's rules that a pawn upon reaching the last rank must be promoted to a bishop, knight, rook, or queen of the same color. Also, please change your avatar.

There's swearing in your avatar. And a good arbiter would make the players go back to the illegal move and have the player choose a piece of his own color to promote to.

No. The rules state that you can't make an illegal play.

I can't speak for FIDE, but I've been a US Chess Federation Tournament Director (includes being arbiter) and according to USCF Official Rules, which must be virtually identical to FIDE rules, if an illegal move was made but 10 or more moves were made after it, the illegal move stands and the game continues!  So it is possible, at least under USCF rules, to illegally promote a piece to the opposite color and have the game ruled legal!  However, moving a piece that's illegally on the board or capturing it also constitutes an illegal move, so it's hard to get 10 moves in without involving the promoted piece!

A pawn MUST be promoted to a piece of the same color and if it's discovered that an illegal position was reached within the last 10 moves and the game was continued as if it was a legal position, the game is restored to the last legal position and continued from there. If the illegal move occurred before the last ten moves, the illegal move stands and the game continues! The following USCF rules apply:

Rule 8F6. Pawn Promotion. On reaching the last rank, a pawn must immediately be exchanged, as part of the same move, for the player's choice of a queen, a rook, a bishop, or a knight of the same color as the pawn.

Rule 11A. Illegal move during last 10 moves. If, during a game, it is found that one of either player's last 10 moves was illegal, the position shall be reinstated to what it was before the illegal move [no time adjustment].

The game shall continue by applying Rule 10, The Touched Piece, to the move replacing the illegal move.  If the position cannot be reinstated [less than 5 minutes on both clocks when no one has to keep score and more than 10 moves after last illegal move is likely], then the illegal move shall stand.

TD Tip: If the moves cannot be reconstructed [go back to the last certain position]. In complicated cases like this the director has a lot of discretion.

Rule 11B. Illegal Prior to Last 10 Moves. ...the illegal move shall stand and the game shall continue.

TD Tip: A King left in check [for multiple moves, means there was a new illegal move made each time the King was left in check].

A final note: I directed tournaments with high school kids where illegal moves were relatively common and scorekeeping was often atrocious.  As the rulebook says, the TD has a lot of discretion and in every case I reconstructed a very probable last legal position from which to resume the game. Most of my illegal situations occurred in cases where a piece was moved incorrectly. For example, after several moves between two new players, someone would notice that both White Bishops were traveling on the dark squares!  Of course, I never had a promotion to the opposite color!

 Wish I would have seen this post was here before posting in the other one.

But FIDE doesn't have a 10 moves ago illegal move rule. If it is found that there was an illegal move, the position just prior to the illegal move must be reinstated.

7.5 a

"If during a game it is found that an illegal move has been completed, the position immediately before the irregularity shall be reinstated. If the position immediately before the irregularity cannot be determined, the game shall continue from the last identifiable position prior to the irregularity. "

In Blitz and Rapid, the rules are a little different and depend on the number of arbiter to games. If there are not enough arbiters, then an illegal move stands once the opponent has made their move unless the players agree to fix the position, without the intervention of an arbiter.  (A.4 b)

I think the USCF 10-move rule is just there as an out in case the prior situation is too confusing to sort out because of kids doing bad scoring, etc.

As I said, I never had to worry about the 10 move rule as a TD - I was always able to construct a last-sure position or one where a bishop on d3 or e2 didn't make any significant difference.

There were a lot of cases of new players drifting into illegal positions in the USCF-rated high school Swiss Tournaments I ran with 30 min on the clock for the rated games and 5 min unrated blitz playoffs to break ties for trophies.  But I have to say I never had an argument from a player when I reconstructed the last legal position - usually the kids were highly embarrassed and said nothing.  And the parents were easy to deal with since very few of them knew anything about chess tournaments.

What would drive me crazy is when some kid would throw a fit because, "My dad says you have to touch the King first when you castle," etc.  And I had several pages bookmarked in the official rulebook because I had to show them dad was wrong about a number of things - usually touch-move related.