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.