As long as your opponent isn't playing white at all:
Celestial Dawn
Color=White Type=Enchantment Cost=1WW MI(R1)/6(R1)
Text(6th+errata): Nonland cards you own that aren't in play are white. Nonland permanents you control are white. Lands you control are plains. Colored mana symbols on all those cards and permanents are {W}. [Oracle 99/06/30]
In addition to being just plain annoying, look at all the rules fun you can have:
It has three effects. First, it changes all your lands into Plains. This can be overridden to change the land type by an effect that enters play after Celestial Dawn. Second, it changes all mana symbols on your cards to white mana symbols. Third, it changes the color of all non-land permanents you control to white. This effect means that even if you Phantasmal Terrain one of your lands to a Mountain, it'll be a Mountain making white mana. Effects that override what color mana a land produces, such as Naked Singularity or Hall of Gemstone can override both effects. [bethmo 96/11/05]
Affects only cards you own and permanents you control. Does not affect mana symbols on cards you do not own and which that are not in play as permanents you control. Therefore, a card Grinning Totem grabs is not affected. [bethmo 96/10/16]
It does affect text 'spliced' into other cards. For example, Zombie Master adds the text "{B}: Regenerate." to zombies. If you opponent had the Zombie Master and you had zombies and Celestial Dawn, your zombies would get "{W}: Regenerate.". [Aahz 96/12/02]
This card affects the color of spells being cast by changing the color requirements in the casting cost. Thus, all your spells with a colored casting cost are cast as white spells. If a spell such as Deathlace changes the color of the spell, the spell will be changed to the appropriate color. [Aahz 97/03/18]
When a card is in play, its "initial color definition" normally determines its color. This definition is defined as "the color(s) of the mana in the card's casting cost". Thus, when Celestial Dawn is in play, all your non-land cards with colored mana costs are white. When it leaves play, the effect ends and all cards revert to their normal colors. This is true even if the cards were cast using white mana while Celestial Dawn was in play. [Aahz 97/03/18]
The effect to turn all your non-land cards (including artifacts) white is a continuous effect that will override the color as set prior to Celestial Dawn's effect. Thus, a Deathlace on casting of a spell or prior to Celestial Dawn entering play will be overridden by Celestial Dawn's effect. [Aahz 97/03/18]
A color change effect which enters play after Celestial Dawn can override the color changing effect of Celestial Dawn. [D'Angelo 97/03/18]
If an effect like Vampire Bats or Dragon Whelp is used while the mana symbols are one color and then they change because this card enters or leaves play, then the previous expenditures are not counted toward the modified limit. For example, if you spent {B}{B} on a Vampire Bats, and then this card entered play, you could spend {W}{W} on it. [WotC Rules Team 97/06/01] [Duelist Magazine #19, Page 31]
Does not affect color words written out instead of being written using the mana symbol. [D'Angelo 97/08/27]
Does not change the type of lands which are not in play. Lands out of play are what they say they are. [D'Angelo 97/08/27]
Any "comes into play" effects of playing a land will not happen since the lands will enter play as Plains. [D'Angelo 99/05/01]
Will not add or remove Snow-Covered nature from a land. See Rule K.21.3. [D'Angelo 98/08/11]
If you use Magical Hack (or a similar effect) to change the word "plains" to a different land type, your lands are all of that new land type, but they still produce white mana. [D'Angelo 00/06/23]