14 September 2006

Pigments, paints, and color mixing wheels

This is the third in a series of posts about color for painters. In the first, I established (to my satisfaction, anyway) that the usual advice on color that you see in most books on painting, based on a three primary color wheel, is not very useful. In the second, I described the Munsell color system, which provides a useful approach to describing color, although it doesn't say a lot about how to mix pigments together.

I know that you've all been waiting with great anticipation for this next post in the series, in which I reveal a simple and comprehensive method that allows you to easily mix exactly the color you want with just a few inexpensive tubes of paint. Alas, I must now confess to you that—so far as I know—such a system doesn't exist. What I can do is to describe some of the issues involved in the complex subject of color mixing and then present several ways to approach the problem.

First, let’s talk a little about paint and pigments. Pigments are colored powders. Some of them are rocks and dirt that have been ground up and purified, some are simple chemical compounds, some are created via complex modern organic chemistry, and some are tiny water-soluble particles that are made suitable for painting by attaching them to larger uncolored particles. Each pigment has a characteristic color (hue, chroma, value) which is the result of the pigment particles absorbing some wavelengths of light and reflecting others. Changing the particle size often changes the color, sometimes radically. Heating many pigments will change the color—burnt sienna is just a cooked version of raw sienna. Changing the medium in which the particle is suspended often changes the color due to a different refraction index—ultramarine blue is much darker in oil paint than in egg tempera. Some pigments come from a single pigment company and in that case all paint makers start with the same raw material. Other pigments are available from multiple suppliers and each may provide a version that is subtly, or not so subtly, different—there are many, many variations on cadmium red. The color of most pigments is different when they are laid on thick (this is called the masstone) and when they are laid on in a thin layer (this is called the undertone). Pigments are usually described as “opaque,” “semi-transparent,” or “transparent.” That refers to how well light passes through the pigment when it is mixed into a transparent binding vehicle. Transparent pigments are much darker and lower in chroma when they are thickly applied than when they are thinly applied. Pthalo blue is a very bright color when spread thinly, but a dark, almost black, blue when it is very thick. Opaque colors are more likely to have a masstone and undertone that are basically the same, although there are exceptions. Transparency varies somewhat from one painting medium to another, because it is a function of the refractive indexes of the pigment and the binding vehicle. With water media, the refractive index of the vehicle changes as it dries, so what looks transparent when you lay it on may be fairly opaque when the paint dries.

Almost no pigments are manufactured for artist’s use; they are made for large-scale industrial purposes, such as signs, cars, house paint, printing, cosmetics, and all of the other industries that use these materials in far greater quantities than artists do. Paint manufacturers buy these pigments and grind them with a binding vehicle and other components to make paint. (In some cases, individual painters may purchase powdered pigments and make their own paint.) A tube of paint may contain only one pigment or several; if adulterating pigments are present in small amounts, it may be considered legitimate not to note that on the packaging. Many paints are mixtures of pigments to make a certain color not obtainable with just one pigment—many companies sell a paint mixture they call “flesh,” for example (usually this means some sort of pinkish attempt at caucasian skin tones). Some companies sell mixtures that are designed to mimic an expensive pigment. So a paint called “cadmium red hue” won’t contain any of the expensive cadmium red pigment, but instead will have a combination of other pigments designed to have similar color characteristics.

So, given these issues, it’s impossible to just develop a simple model of color mixing; every artist uses different pigments, in different media, made differently by each company (and, perhaps, formulated differently from one batch to another). And every artist applies paint differently.

Even if you could correct for all of these factors, there are other challenges. Let’s look at one attempt to produce a color mixing system for artists: the Quiller wheel. Stephen Quiller, a working artist (mostly in watercolor and acrylic) has designed a color wheel to assist artists in figuring out how to mix color. A large number of common colors are placed in various positions around the wheel. Where the color is placed depends on (1) the color’s hue and chroma; and (2) the color’s mixing complement (which is directly across on the other side of the wheel). Color placements are a matter of Mr. Quller’s judgment and extensive mixing experience, not any sort of mechanical measurement. Lower chroma colors are closer to the center of the wheel, while higher chroma colors are closer to the outside. So if you are using the Quiller wheel, and you want to mix a particular color, you would find the location on the wheel of the color you want to mix, then mentally draw a straight line between two actual colors. In theory, any color on that line can be mixed with those two colors. You can therefore find a mixture that is the one you want.

A color mixing wheel such as this will often get you into the right ball park when mixing colors. Unfortunately, it has some problems. First, there are the issues of paint variability discussed above. One cadmium red light is not necessarily the same as another. Second, there is the problem of hue shifts. Because of the physical properties of pigment, when two colors of paint are mixed in different proportions, the hue doesn’t always follow a straight line that can be plotted on a wheel. The hue instead follows a curved path that is not predictable just from knowing the colors of the two paints being mixed. This curved path might mean that the color you want cannot be mixed from the two colors you’ve selected, even though it looks like that should be possible from looking at the color wheel. Another problem with a mixing color wheel is this: paints can have multiple mixing compliments. A complimentary mixing pair, you will recall, is two paints that, in some ratio, can be mixed to form a grey color with 0 chroma (or very, very close to 0). A mixing color wheel attempts to place complimentary pairs directly opposite each other on the wheel. But because in real life a pigment may have more than one mixing compliment, its actual location on the wheel is an arbitrary choice. Since the wheel is trying to do two things that are not always compatible (show the appropriate location for mixing and show complimentary pairs), it represents a compromise. It's a set of guesses designed to help the artist get close to what he or she is looking for. It’s not a scientific instrument; it’s a compendium of rules of thumb, and those rules of thumb are often wrong. Another problem is that the wheel shows purples and yellows at opposite sides of the wheel, indicating that they are complimentary mixing pairs. But the reality is that most yellows and most purples don't have a mixing compliment, so the wheel deceives you when you try to mix those colors together.

There’s another problem with mixing wheels. They help you get into the right ballpark with the hue you are looking for. They can also give you an idea of what chroma any given mixture will be, since you should be able to find the approximate chroma of a mixture by finding where it lies on a line between two mixing pairs; the closer to the center of the circle, the lower the chroma. But what if you mix the perfect hue, and it’s the right color. How do you get the value you want? If you want a darker color, mixing with black will reduce the chroma drastically and will often shift the hue. If you want a lighter color, mixing with white will also reduce the chroma and will often shift the hue as well. So you may have the perfect mixture, except for value, and still not know how to get the final color you want. The color mixing wheel doesn’t get you to the finish line.

That all being said, the Quiller wheel (or something like it) is a useful tool that can be helpful, especially for beginners, in figuring out the basics of color mixing. There are other mixing wheels out there, sometimes with fancy sliders that you can turn around a cardboard wheel. There is also color mixing software that attempts to do the same thing. They all have significant limitations when it comes to making subtle mixtures of actual paints.

So what’s a poor painter to do?

Well, since many real-world painters manage to get good results with color, it is obviously possible to do so. In the next post, I’ll discuss several strategies for color mixing.


Ujwala said...

I'm a beginner and following this series of posts with great interest. A fellow WetCanvaser.

David said...

Thanks, Ujwala. I'm glad you found the site helpful.

I checked out your web log. It has some useful content. Your work is very clearly influenced by Modigliani.