The phrase "grey line" refers to the boundary between night and day, where the sky looks a bit gray. It's the time when the sun is rising or setting, and the darkness of night meets the brightness of daylight.
Sentences with «grey line»
All seats are covered with black leather with grey lines on the edges. (thesupercars.org)
The white line of quartz stone is replicated in the white or grey lines of her paintings. (roberthenrycontemporary.com)
The silver grey lining in the stack is blue - green algae and blackberries. (dailyburn.com)