14 October 2006

Practical Color Mixing 2: Hue

In the previous post in this series on color mixing, I talked about how to mix the right value. Here I’ll talk about hue.

Identifying hue

Before you can mix the right hue, you need to figure out what hue you want to mix. That’s often kind of hard, especially with the dull, low-chroma colors that predominate in most of the visual world. Look around you. What color is the wall? A yellow-green? Or is it more of a middle yellow? How about the cable leading to your monitor? Is it black, or some very dark greyish color? If so, is it a warm dark grey or a cool dark grey? What about the shadow falling on the floor from your desk? You get the picture.

When I’m standing around waiting for something I find myself trying to identify the color of various things around me. And not just the color of the thing (the “local color”) but the color of the shadow side, the light side, and so on. I think about value and chroma as well as hue, but the hue is often the hardest to figure out. As with any other attribute of color, it’s best to think in terms of comparison of one color with others around it. Once I think I know what the color is, I consider how I would mix it. That seems like a pretty geeky thing to do (and it is) but it’s a useful exercise. We think we know what color things are, but while it’s easy to say that the sky is blue, it’s a bit more of a challenge to determine that the part of the sky beyond those clouds is a green-blue, while the sky in between the clouds is slightly purple.


So once you know what hue you want, how do you get it? Well, if the hue is pretty close to a paint that’s already on your palette, you might be able to just tint it in one direction or another. Say you want a violet blue and you have ultramarine on your palette. Without having to think about color wheels or anything complex like that, you could simply add just a bit of a more purple color, such as dioxazine violet. That may well get you where you want to go, simply and easily. In doing this, the thing to realize is that any given paint can only go in two directions from where the hue is right now—either clockwise or counterclockwise on the color circle. Ultramarine blue can be made more violet or more green. That’s it. Cadmium orange can be made more yellow or more red. If you’re just trying to nudge the hue around a little bit, all you have to decide is which direction to go and select a color next door on the color wheel to move it in that direction.

Using a mixing color wheel

The nudging strategy is great for small adjustments, but it starts to fall apart when you need to make a hue that isn’t close to one of the paints you already have. At that point, it’s useful to go back to the concept of a color mixing wheel. As I’ve pointed out previously, a color mixing wheel does not provide a precise guide to what you will get with any two pigment mixtures. Individual pigments are simply too idiosyncratic in their mixing properties to allow any kind of absolute prediction of how they will behave when mixed. But a color mixing wheel will help you to get into the approximate ballpark, at which point you will be close enough to use the nudging aproach described above.

Steven Quiller sells a useful color mixing wheel. Bruce MacEvoy at Handprint has a somewhat different one that you can print out for free (it's designed for watercolor, but I have found it to be reasonably useful for other media as well).

Say you need to mix a yellowish green, but don’t have anything close to that on your palette. If you look at a color mixing wheel, the two colors on either side of green are blue and yellow. As we all know, you can mix a green from blue and yellow, and if you adjust the proportions correctly, you can pretty easily get a yellowish green. If you have any set of paints that are selected to fall at reasonable intervals across the color wheel (at least a cyan, magenta, and yellow), you can mix any desired hue using two or three paints.

A traditional color wheel is set up so that all of the colors on the outside are as high in chroma as that hue goes (without regard to value). On the inside are less chromatic colors, arranged so that the closer to the center they are, the lower the chroma. The basic mixing procedure goes like this: (1) identify a point within the color mixing wheel that represents the desired hue and chroma; (2) look for one or more lines between two paints that pass through (or near) the color you are trying to match; and (3) consider whether a third paint (typically one on the opposite side of the wheel from the desired color) might be needed to adjust the chroma downward. If the paints have equal tinting strength, you can figure out approximately how much of each paint you will use, based on where the desired color falls on the line between the two paints being mixed. If one paint is stronger, you’ll need to adjust accordingly to account for that. As a general rule, put out some of the weaker paint and add the stronger paint to it. Alternately, put out the paint you will use the largest amount of and add the other paint to it. Add in small increments at a time—I find myself overshooting frequently if I’m not careful.

With oil paint, it's best to mix with a palette knife rather than a brush. Once you're used to it, the knife is faster because you can clean it so quickly, and your paint piles don't become contaminated with other pigments.

Coordinating hue and chroma

Notice that if you draw a straight line between any two colors on the outside the wheel, every point on the line represents a lower chroma than those two colors. So mixing tends to reduce chroma. As a general rule, any mixture is duller than the brighter of the two paints being mixed, and often duller than either one. There are a very few exceptions (some warm pigments become a little more chromatic when mixed with each other and some cool pigments become more chromatic when mixed with a small amount of white), but chroma reduction is the usual effect of paint mixing.

Often, it’s useful to have mixing reduce chroma, because in realist painting you are frequently trying to mix a color that is duller than the tube paints you have available. If you’re trying to mix a flesh tone with bright cadmium colors, for example, any reduction in chroma is welcome (you've probably seen bad amateur portraits with bright orange flesh tones). But there are times when you are trying to mix a high-chroma color, and in that case the chroma reduction from mixing can be frustrating. Because of this effect, it’s often a bad idea to just muck around with paint, hoping to get close to the color you’re looking for. Every paint you add to the mix cuts the chroma down, so after awhile you are just mixing paint into a sort of nondescript grayish color—i.e., you're mixing "mud."

It’s is much better to decide what color you want, choose two or three paints that you will use to get that color, and try to stick with those. Minor nudging with other paints is OK, but if the mixture goes radically in a direction you didn’t expect, don’t keep throwing additional paints in, hoping you’ll eventually get to your desired color. Once you have mud, just scrape it off your palette (or use it as the basis of some nondescript color you need elsewhere) and start over. Take a step back and think again about what you’re trying to accomplish and how you’re going to get there.

As noted above, many pigments fail to follow a straight line on the color wheel when mixed. In particular, some paint mixtures follow a circular mixing line. That means that, while the wheel predicts the correct hue, the chroma is higher than expected (this is particularly common with greens). In that case, go ahead and mix the hue you want, then tone it down (I’ll talk about adjusting chroma in the next post in this series).

Coordinating hue and value

We’ve talked about getting to the right hue and chroma, but what about value? It would be easier to make this color mixing thing work if there were only two parameters to worry about, and many color mixing books kind of pretend that’s the case. When I talked in my last post about getting the value right when mixing, I suggested that the first thing you do when trying to make any particular color was to first mix the colors you are working with to the correct value. You can then mix them together and get the hue and value you are looking for—because the paints are already at the correct value, you don't have to think about that factor any more, greatly simplifying the problem you're trying to solve. Mixing value first usually works, except when you can't get the right chroma and value because the white paint is pulling the chroma down too far. I’ll talk about strategies for dealing with that problem when I discuss chroma in the next post. Under most circumstances, the “mix the value first” rule makes color mixing much easier to control.

Warmth and coolth

The idea of warm and cool colors has many implications for composition that don’t belong in a discussion of color mixing. Warm colors are generally thought to include red, yellow red, and yellow, while cool colors are thought to include blue green, blue, and purple blue. (I’m using Munsell hue terminology here.) In between colors include green, green yellow, purple, and red purple (some people would label green and purple as warm and green yellow and red purple as cool). There are some aspects of the warm/cool division that are useful to include in a discussion of color mixing.

If you reduce the chroma of a warm color, it appears less warm (raw umber is less warm than cadmium orange); if you reduce the chroma of a cool color, it appears less cool. The most chromatic warm colors are much higher in chroma than the most chromatic cool colors. Compare, for example, cadmium yellow light (a high chroma yellow) and pthalo blue (a high chroma blue). The yellow is much higher in chroma than the blue. Not only that, the yellow is also much lighter in value than the blue. Warm pigments can be very light (high in value) at high chromas. Cool pigments are much darker at their highest chroma. Adding a lot of white to a cool pigment, to bring the value up near to that of cadmium yellow, decreases the chroma still further. You can’t have a high chroma, high value cool color—the physics of light don’t allow it. These kinds of differences are why the Munsell color space is shaped like a bumpy, irregular cylinder.

Because of this effect, high chroma warm colors have more overall punch than high chroma cool colors. Many art books tell you that warm colors advance and cool colors recede. That’s wrong, although it has a germ of truth. The eye looks for contrast. In most paintings, chromatic colors have more contrast with their surroundings. Higher value colors also have more contrast. Because warm colors are more chromatic and higher in value, they have more contrast, so they jump forward. If you drop the chroma and value of warm pigments to match those of cool pigments, they become brownish and don’t have any extra punch at all.

The warm/cool contrast can also be useful when you're trying to figure out what hue something has. It can be easier to ask whether a hue is warmer or cooler than a color near it than to try to figure out its hue directly. Any hue can be shifted either clockwise or counter clockwise on the hue circle. You can think of this as shifting warmer or shifting cooler. For example, a yellow can shift toward green (cooler) or toward red (warmer). That is not to say that red is warmer than yellow (different people have different opinions on that issue), but that shifting toward red is shifting away from a cooler color (green is definitely cooler than red), so it’s useful to think of that as “warmer” in this context. Similarly, a purple can be shifted toward red (warm) or toward blue (cool). As you are painting, you can think in terms of these comparisons. Is the hue on the light side of an object warmer or cooler than the hue on the shadow side? That comparison is an easier task than determining the absolute hue of the light side.

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