Color solid
Painters long mixed colors by combining relatively bright pigments with black and white. Mixtures with white are called tints, mixtures with black are called shades, and mixtures with both are called tones. See Tints and shades.
Optimal color solid plotted within the CIE L* a* b* color space (left or top), and within the CIE L* u* v* color space (right or bottom), both with D65 white point. Because they are (approximately) perceptually uniform color spaces, the optimal color solids have an irregular, not spherical shape. Notice they have two sharp edges, one edge with warm colors, and the other edge with cold colors.
A color solid is the three-dimensional representation of a color space or model and can be thought as an analog of, for example, the one-dimensional color wheel, which depicts the variable of hue (similarity with red, yellow, green, blue, etc.); or the 2D chromaticity diagram (or the color triangle), which depicts the variables of hue and spectral purity. The added spatial dimension allows a color solid to depict the three dimensions of color: lightness (gradations of light and dark, tints or shades), hue, and colorfulness, allowing the solid to depict all conceivable colors in an organized three-dimensional structure.