What do you think is the color of ‘sour cream’? Why do you think the poet has used this expression to describe the classroom walls?

Explanation

The walls are sour meaning they don't fit into the taste of the classroom. They don't justify or are unpalatable to what is inside. Here, sour refers to the color of sour cream - off-white or creamish. They have posters in donations given by others. The color of the walls is dull, like the dejection in the eyes of the children.