darken

This term describes a transition toward darkness, whether through the physical removal of light or the intensification of a pigment. It carries a neutral tone when describing environmental changes, such as the onset of twilight, but shifts toward a negative connotation when applied to human emotion or facial expressions. When used to describe a mood or expression, the word suggests a sudden clouding of temperament, often implying a shift toward anger, sadness, or suspicion. This usage transforms a visual property into a psychological state, linking the absence of light with the absence of joy or openness.