«فِي لَيْلَةٍ أَدِيمُهَا ذُو لَوْنَيْنِ»
On a night whose outer skin seemed to have two colours.
Adīm properly refers to a hide or its surface. By giving night a skin, the phrase makes darkness and moonlight feel like contrasting colours on a single surface.
The “skin” is imagery, not a literal object. The description establishes a visually memorable night while preparing the reader for a surprising event.