Fig. 1.

Diminishing returns imply that large color differences appear less than the sum of their parts. This image is a purely figurative illustration of how this phenomenon could potentially occur, even though this specific geometry is not suggested in this paper. If an isoluminant plane through color space (Upper) would, for example, have the shape of a curved two-dimensional submanifold embedded in a 3D space (Lower), then the 3D Euclidean metric would produce the inequality of diminishing returns.