Celestial Dawn
{W}{W}
Enchantment
Nonland cards you own that aren't in play, spells you control, and nonland permanents you control are white. Lands you control are Plains. Spells and abilities you control produce white mana instead of any other color. You may spend white mana as though it were mana of any color.
I would say no. From the Comp. Rules glossary:
Owner
The owner of a card is the player who started the game with that card in his or her deck or, for cards that didn’t start the game in a player’s deck, the player who brought the card into the game. (Legal ownership is irrelevant to the game rules, except for the rules for ante.)
If you use an effect to bring a card into the game, you are by definition its owner. Since there is no such thing as "between zones", the moment a wished-for card enters your hand, it becomes white. However, I do not think that Celestial Dawn makes your entire sideboard (or collection, for casual) white. Here's a hypothetical card, templated from existing wishes. Is this kind of what you're getting at?
Alabaster Wish
{W}
Sorcery
Choose a white card you own from outside the game, reveal that card, and put it into your hand. Remove Alabaster Wish from the game.
Interesting. Now that I read it again, it seems the Wishes use the word "own" to mean "legal owner," which is clearly not what the definition is according to the rules. Golden Rule wins, though. Still, there's no reason to think that Celestial Dawn also breaks the rule that way, since there's no compelling reason for it to do so.
