user4136: Is it legal to underpromote a pawn to opponent's pieces? For example, in the below position, promoting White's pawn to a Black knight will be a win.
[FEN "r7/kPR5/8/K7/8/8/8/8 w - - 0 1"]
[Title "Underpromotion"]


user291: No, it is not.
FIDE Laws of chess, clause 3.7e:

When a pawn reaches the rank furthest from its starting position it
  must be exchanged as part of the same move on the same square for a
  new queen, rook, bishop or knight of the same colour.


4136: <p>Is it legal to underpromote a pawn to opponent's pieces? For example, in the below position, promoting White's pawn to a Black knight will be a win.</p>
<pre><code>[FEN &quot;r7/kPR5/8/K7/8/8/8/8 w - - 0 1&quot;]
[Title &quot;Underpromotion&quot;]
</code></pre>
 291: <p>No, it is not.</p>

<p>FIDE Laws of chess, clause 3.7e:</p>

<blockquote>
  <p>When a pawn reaches the rank furthest from its starting position it
  must be exchanged as part of the same move on the same square for a
  new queen, rook, bishop or knight <strong>of the same colour</strong>.</p>
</blockquote>
