purity

"Pure color" refers to a color produced by homogeneous spectral light or a color with a purity similar to that of the physical spectrum. Purity in color can be measured using tristimulus values, which are based on matching a color against red, green, and blue primaries. These values, represented as x and y coefficients, determine a color's chromaticity. A purity of 100 percent signifies a pure spectral color, while 0 percent represents achromatic colors like white, gray, or black.12

Many compounds are colorless when pure because their electrons are paired, requiring high energy for excitation. However, transition metals like iron, chromium, nickel, cobalt, and manganese contain unpaired electrons, readily forming excited energy levels that absorb photons and produce color. For instance, chromium impurities in aluminum oxide create the red color of ruby.3