Complementary Color Finder

A complement in this tool is defined arithmetically: convert the input to HSL, add 180 degrees to the hue with wraparound, and keep saturation and lightness untouched. The split complements are the same rotation offset to 150 and 210 degrees, which lands them on either side of the true opposite. Holding saturation and lightness constant is what makes the pair look like siblings rather than unrelated swatches, but it has a direct consequence that the contrast readout exposes. Two colors at identical HSL lightness usually have similar relative luminance, so a true complementary pair commonly measures somewhere between 1.1 to 1 and 2.5 to 1, far under the WCAG minimum of 4.5 to 1 for normal body text. The pair is an accent relationship, not a text relationship, until you deliberately break the lightness parity by taking one side down to roughly 25 percent lightness or up past 85 percent. The second thing to expect is that the answer will not match a painter's intuition. On the RGB and HSL wheel the opposite of red at hue 0 is cyan at hue 180, whereas the traditional RYB pigment wheel taught in art classes puts green opposite red and orange opposite blue. Neither is wrong; they describe different physical processes, light addition versus pigment absorption. For accent palettes the 60-30-10 allocation is the usual working rule, giving the base color about 60 percent of the surface area, a supporting tone 30 percent, and the complement only 10 percent, because sustained equal-area complementary contrast produces visible edge shimmer and eye fatigue.

Opposition on a color circle dates to Isaac Newton, whose 1704 diagram in Opticks placed hues around a ring so that mixtures could be found by geometry. The concept became a working method through Michel Eugene Chevreul at the Gobelins tapestry works in Paris, who was assigned to investigate why certain dyed yarns looked weak in finished tapestries. He concluded that the fault lay with adjacent colors rather than with the dyes, and published the law of simultaneous contrast in 1839 along with a 72-part chromatic circle. Ewald Hering proposed opponent process theory in 1892, arguing that vision encodes red against green and blue against yellow, which explains why afterimages appear in complementary hues. Chevreul reached painters quickly, reinforced by Ogden Rood, a physics professor at Columbia whose Modern Chromatics of 1879 was read closely by Georges Seurat and Paul Signac and shaped the complementary juxtapositions of pointillism. Albert Munsell built the idea into measurement in 1905 by defining complements as the pair of hues that mix to a neutral gray under his notation. The arithmetic 180 degree rotation used by software is a much later convenience, arriving with HSL and HSV color pickers in computer graphics after Alvy Ray Smith described the cylindrical models in 1978. The contrast ratio reported alongside the complement reflects a modern requirement rather than a historical one, coming from success criterion 1.4.3 of the W3C Web Content Accessibility Guidelines.