Undertones of blue or green make it a cool color. For the most pleasing results, your curtain colors should be warm if your walls are a warm gray, and cool if the walls are more on the cool side.

Cobalt-blue curtains hang in place of walls in a small apartment in Kyiv renovated by Ukrainian architecture and interiors studio Ater Architects. Called EGR Apartment, the 65-square-metre space ...