Hostile Desert is colorless because it has no mana cost (C.R. 202.2, especially C.R. 202.2b), and its second ability doesn't give Hostile Desert any colors when it resolves. (Compare Hostile Desert with Svogthos, the Restless Tomb. Note that Ghostfire Blade's equip ability can target any "creature you control", even if it isn't colorless [C.R. 702.6a].)
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Want to play a UW control deck in modern, but don't have jace or snaps?
Please come visit us at the Emeria Titan control thread
EDIT (Feb. 5): Correctness edit.
Nexus MTG News // Nexus - Magic Art Gallery // MTG Dual Land Color Ratios Analyzer // MTG Card Drawing Odds Calculator
Want to play a UW control deck in modern, but don't have jace or snaps?
Please come visit us at the Emeria Titan control thread