Etymology: Combines hole 穴 with work 工. Originally, this character meant to work to dig a hole in the ground and place a covering over it to form an ancient dwelling place.
The domed roof became associated with open space, which over time extended to mean upper space and then then sky itself.
Mnemonic: WORK TO OPEN HOLE IN ROOF TO SEE EMPTY SKY
空気 (kuuki) air
空色 (sorairo) sky-blue
(Based upon Henshall, 1998)