This is the 340th most frequent Japanese word.
青
Blue; green.
Here, '青' (あお) refers to the blue color of the sky, describing its appearance.
空が青いです。
The sky is blue.
In Japan, '青' (あお) is also used to describe the green color of traffic lights, which is a cultural nuance.
信号が青になったので進みましょう。
The traffic light turned green, so let's proceed.
In this context, '青' (あお) specifies the blue color used in the painting, focusing on its aesthetic quality.