If there are 3 primaries, why don't we just buy 3 tubes of paint to mix with? This is the question I often receive in color theory. The problem with paint is that it isn't the same as light. It's impure. It would be wonderful if we could have paint the color of butterfly wings or peacock feathers, beautiful, ethereal, ephemeral things. But we're stuck with what comes out of the earth or what is created in a chemical lab. Our paint is colorful mud. And the more it's mixed together, the muddier it gets.
Light comes down from the sun or a light source, and the rays of light stike a surface; some of the colored rays are absorbed into the surface, and the others are reflected back. The color of the reflected light rays determines what we see. Each paint pigment has different absorption and reflectant properties. When you mix two paints together their absorption characteristics are combined and less light is reflected back. This is why mixtures are always duller than their parent colors. The brightest colors are those that come straight out of the tube. Mixed secondaries and tertiaries will never be as bright as a pigment secondary or tertiary. So if you need to paint a lot of green, orange, or purple- go buy a tube of paint!
This week my students created grids of color mixtures. The dots along the edge represent each warm and cool primary, with additional rows for white, black, and burnt umber. It's interesting to see how varied their results were depending on which order they placed their colors, and how balanced their mixtures were. It is plain to see that different starting pigments create vastly different results. Some of the individual squares are brighter than others. Some are extremely "muddy" looking.
Sometimes you want a duller color! Instead of dulling down a bright paint by mixing in the complement, you can start off your mixture by using the pigments that are farther apart on the color wheel.
The image below shows 2 attempts to match a color chip. It's harder than you think! Hopefully this practice in color mixing will help make painting a canvas easier- you can spend less time deciding how to mix the paint and more time actually painting when you get familiar with the possibilities of your pigments.