Gradient – A gradual increase or decrease of a visual quality such as size, orientation, brightness, or color (see Descriptors). The intervals of a gradient must be significant enough to discriminate change and must consist of sufficient intervals to sense a transition; three shapes will typically be insufficient to indicate a gradient.

Brightness Gradient

Color Gradient (or Hue Gradient)

Size Gradient

Size Gradient (the size gradient also indicates the depth cues of linear perspective and known size/scale)