The effect to turn all your non-land cards (including artifacts) white is the effect of a static ability. Thus, a color change on a permanent prior to Celestial Dawn entering the battlefield will be overridden by Celestial Dawn’s effect.

Celestial Dawn does not change the type of lands which are not on the battlefield. Lands not on the battlefield are what they say they are.

Will not add or remove Snow Supertype from a land.

You may use a different color mana than the color required for spells and abilities that require a specific color. If you do, and the spell or ability checks the actual color of the mana, it can tell the difference.

If you pay white mana to activate an ability that can only be activated by spending another color of mana, then any limitations on activating the ability (such as the number of times it can be activated each turn) still apply as if you spent the proper color.

Celestial Dawn no longer changes mana symbols on cards.

Mana produced by spells you control and nonland permanents you control isn’t affected by Celestial Dawn. Llanowar Elves will produce , for example. However, any colored mana may be spent only on the colorless part of costs.

Land cards you own that aren’t on the battlefield aren’t changed to Plains.

If you have nonwhite mana in your mana pool when Celestial Dawn enters the battlefield, it remains there but you can spend it only on the colorless part of costs.

“Enters the battlefield” triggered abilities of lands you play won’t trigger since the lands will enter the battlefield as Plains. Effects that modify how those lands enter the battlefield, however, will still work. For example, if you play a Dimir Aqueduct, it will enter the battlefield tapped as a Plains, but you won’t return a land to your hand.

If you use a text-changing effect such as Mind Bend on Celestial Dawn to change the word “Plains” to a different land type, your lands are all of that new land type, and they will produce mana of the appropriate color for that type.

If a permanent enters the battlefield while Celestial Dawn is on the battlefield, and then Celestial Dawn leaves the battlefield, the permanent’s color will revert to the colors in its mana cost.