What element of design refers to the lightness or darkness of a colour?

The element of design that refers to the lightness or darkness of a colour is value. Value describes the range of tones from the lightest (white) to the darkest (black), including all the gradations in between. Every colour has a value: for example, yellow in its pure form has a high value (it is light), while purple has a low value (it is dark).

Understanding value is essential in art because it allows the artist to create contrast, depth, form, and atmosphere in a composition. By varying the values in a drawing or painting, an artist can make objects appear three-dimensional and create a sense of light falling on surfaces.

Hue refers to the name of a colour itself (red, blue, green), not its lightness or darkness. Texture describes the surface quality of an object or material (smooth, rough, glossy). Tone is sometimes used informally as a synonym for value, but in the formal classification of the elements of design, value is the standard term for the lightness-darkness scale of a colour.