watercolor mixing complements

Subtractive complementary colors or mixing complements are two paints, from roughly opposite sides of the color wheel, that can mix to make a pure gray or achromatic color. These paint pairs are invaluable for mixing dark neutrals, and as signposts to color design decisions. This page presents the watercolor mixing complements I identified by extensive testing.

Method. As there are many fewer pigments in the blue or green side of the artist's color wheel than on the red or yellow side, I first selected the 21 most important "cool" single pigment paints (including two shades of phthalo and cerulean blue, and two convenience greens). I then chose a comprehensive selection of 50 "warm" mixing complement paints, from violet red to green yellow, to mix with them.

For each "cool" (blue or green) target paint, I first selected 6 to 10 of the "warm" (red or yellow) paints that were opposite the target paint on the color wheel and therefore most likely to be mixing complements for the paint. I made four or five mixtures with each warm/cool pair, varying the proportions of the mixtures to bracket both sides of a visual gray, and applied the mixtures to a "bright white" watercolor paper (Cheap Joe's Kilimanjaro).

These mixtures (over 700 in all) were measured with a Spectrolino spectrophotometer, and the results plotted (using Excel) on the CIELAB a*b* plane. Points from the same pigment pair were connected with a line, and these mixing lines were used to judge whether a mixing pair passed through the achromatic point (and were therefore true mixing complements) or missed it on either side (near mixing complements).

Using these charts as a guide, I then mixed and measured all additional "warm" pigments from the set of 50 that were located on the artist's color wheeel next to the true mixing complements I had already identified, to ensure I tested all possible mixing complements. (This resulted in another 800 paint mixtures, over 1500 in all.) The example at right shows the final plot for ultramarine blue, PB29 (with the a+a/- dimension reversed left to right to match the artist's color wheel). Note that 19 mixing complements were tested, though only 7 are listed in the table below.

Any mixing line that passed within a minimum chroma of 5 or less (light gray circle) is a "near gray" neutral that most viewers would find visually equivalent to a pure gray. Any mixture with a minimum chroma of 2 or less (dark gray circle) is considered a pure gray. In the diagram (above), there are 4 near gray neutrals and 3 pure gray neutrals with ultramarine blue; all the other paint mixtures stray too far from the achromatic point. Over 170 unique pigment pairs produced pure gray or near gray mixtures.

Results. Why have I presented the results as a big list rather than as a color wheel? Because it is not possible to make a consistent mixing color wheel, for reasons explained here.

To demonstrate the basic problem, I've made a visual table of my mixing results by listing warm hues, in order of hue angle (from cadmium lemon to ultramarine violet), in the rows down the left side, and the 21 target cool hues (from ultramarine violet through sap green) in the columns across top. Each cell in the table represents the unique mixture of a par of warm and cool paints. Then I used a black rectangle to mark the paint combinations making "pure gray" mixtures, and a gray rectangle to mark the "near gray" mixtures (as defined above).

The black and gray cells do not form a solid, straight diagonal through the table, but are broken up and scattered irregularly by substance uncertainty. This should help convince you that subtractive color mixing is really messy! In fact, you can't reduce material color mixtures to a color wheel. Mixing complements have to be defined in terms of specific pigment pairs, not abstract "color" contrasts.

The measurements are summarized in the table below as follows:

• A listing of all "cool" target pigments, including color index name and manufacturer. (Some paints, such as cerulean blue, vary widely across manufacturers.) Convenience green paints are listed in italics.

• A listing of all the "warm" mixing complements for each target pigment, with the manufacturer and color index name. The "pure gray" complements (minimum chroma of 2 or less) are shown in boldface type; the "near gray" (minimum chroma of 5 or less) below them in regular type.

• The hue bias for the "near gray" mixtures, as a deflection toward the yellow, magenta, blue or green sides of the hue circle.

• The approximate value range of the neutral color, averaged across all test mixtures. The higher the value range, the darker the color (carbon black watercolor pigments reach a value range of 70 to 75).

These complements may or may not apply in other media, such as acrylics or oils, which use a different vehicle that can change the reflectance attributes of the pigments. For the pigments or paints footnoted (^b), the mixing behavior in watercolors may vary by manufacturer.

A thumbnail image of an artist's color wheel shows the location of each cool target pigment and all the warm neutralizing pigments listed for it. These suggest the wide range of warm pigments that can neutralize a single cool pigment color.

I haven't indicated the mixing complements that show significant drying shifts in hue or lightness, or that quickly separate when mixed in diluted concentrations, and are therefore harder to mix to a true gray. These shifts are noticeable in the pigment pairs with very different drying shifts, and in pairs having different tinting strengths (imbalanced mixing proportions).

Two comments. The liquid mixtures of cadmium and phthalo pigments are deceptive, as the cadmiums tend to sink quickly in the mixture, making it appear cooler than it actually is. This is a general mixing hazard with any combination of synthetic organic and inorganic (mineral) pigments. In addition, many mixtures of cobalt pigments with synthetic organic or "earth" (iron oxide) pigments will separate if applied as a juicy brushstroke, which will look like a complex or mottled patch of two distinct colors, even though the table indicates the colors average to a near or pure gray. Sometimes these separations are a nuisance, but often they can be used to produce beautiful textural effects. Always stir vigorously to judge the color when mixing, and evaluate the mixture as a test swatch on scrap paper before painting.

Last revised 08.I.2015 • © 2015 Bruce MacEvoy