Can I Wish for a card outside my Commander's colors?
Wishes are non-functional in Commander, unless your group specifically allows them and defines where they work.

The comp rules define the basics of the format but mostly point you to mtgcommander.net for the up-to-date official definition. Per mtgcommander.net,


  Abilities which refer to other cards owned outside the game (Wishes, Spawnsire, Research, Ring of Ma'ruf) do not function in Commander without prior agreement on their scope from the playgroup.


Basically the Rules Committee's reasoning was that using wishes in their original, casual-play way (searching your entire collection) wastes too much time in play1. So they used to restrict you to searching your sideboard, which is how wishes work in competitive MTG. However, Commander's sideboards are "optional" and Sheldon has generally come out against them, so the default is that wish cards don't work at all.

Sideboards still have to follow all the normal deck-construction limitations, like in any other format; for Commander, this means you can't have break the one-of restriction or violate color identity just because a card is in the sideboard. Thus, if you group agrees to play with wishboards, you'll still be limited to your commander's color identity.

(Note also that the official sideboard size is 10 cards, not 15 like 60-card constructed.)



1 &ndash; Which is also why Shahrazad was finally banned after some time as the only Vintage-banned card legal in Commander.