The value of X is calculated only once, as Aurora Awakener's triggered ability resolves.

An Aura being put onto the battlefield this way can't enchant anything else that is being put onto the battlefield at the same time.

Lands are normally colorless permanents, even if they tap for mana of a certain color.

The colors are white, blue, black, red, and green. Colorless is not a color.

If an Aura is put onto the battlefield without being cast, the Aura's controller-to-be chooses what it will enchant as it enters the battlefield. An Aura put onto the battlefield this way doesn't target anything (so it could be attached to an opponent's permanent with hexproof, for example), but the Aura's enchant ability restricts what it can be attached to. If the Aura can't legally be attached to anything, it remains in its current zone.