Generally, You have to be able to make 3 decisions about a color to mix it precisely. You have to decide on the target color's (1) hue, (2) value, and (3) saturation.
For definitions and illustrations explaining hue, value, and saturation, please see the attached reference sheet containing the color wheel. The information compiled on the sheet is provided free of charge from Wikipedia and also borrowed, with much gratitude and written permission, from artist Malane Newman's website: http://www.malanenewman.com/color_theory_color_wheel.html
Mixing in Manageable Steps:
Please refer to the attached handout showing the apple.
The apple reference sheet will illustrate the steps outlined below. Please feel free to retrace the steps using the free WetCanvas interactive
color mixing palette http://www.wetcanvas.com/colormixer/colormixer.php
I used the WetCanvas virtual palette to process the mixtures and I also borrowed the apple reference image from the Reference Image Library at WetCanvas. (Thanks to Sketchophrenia from Wetcanvas for contributing the reference photo.)
Let's begin by first isolating and then mixing a specific color on the apple. In practice, our first color decision in matching a target color is to decide on the hue and to mix it.
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Step 1 Hue
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We first pick the tube color that most closely matches the color that we wish to mix. Then we decide if we need to modify that hue.
In the case of the apple, we decide alizarin is the closest hue resembling our target color, but not quite close enough. The section of the apple we are considering is just a little more purple looking than pure alizarin. How do we achieve that slight shift toward purple? Mix in some blue into our alizarin pile to bend that color toward purple. (Red + Blue= Purple.) If we accidentally add too much blue, we just need to add some more alizarin
back into our mixture to reduce the proportion of added blue. In real life we should be testing our mixture by painting a swatch of it on scrap paper and holding it up to our target color to compare.
After approximating our hue, our next step is to consider the target color's value. (In practice, if you are having trouble determining the closest top level hue of your desired target color, hold up your color wheel next to the target. Ask yourself, "Is this target color more red, yellow, blue, purple, orange, green?")
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Step 2 Value
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Value is the relative degree of lightness or darkness of a color.
In practice, Our next step after mixing the proximate hue of our hypothetical apple is to adjust the value by deciding if our mixture from step 1 is lighter or darker than your target color. If the red we mixed is appearing darker than what is observable on the hypothetical apple, we need to lighten our mixture. We can lighten by either adding in
either white, or yellow, or both at the same time. Adding just white to lighten our mixture will not only lighten it, but will push the color pile a little towards blue. Adding just yellow will not only lighten our mixture but will also push the color pile towards being yellow (orange.)
Adding white and yellow at the same time will lighten the value without pushing the pile too far blue or yellow. (Always consider using yellow or white plus yellow to lighten a colors value.) So, in practice, we just need to add a little white, or yellow, or both to our red hue mixture to lighten it up a little. In the case of they hypothetical apple, we decide it's safe to only add white to lighten the value. (Since our target color is bluish red we don't want to lighten our mixture with yellow, which will bend our color towards yellow.) (Had we been painting the apple in shadow we might have needed to darken our red/purple mixture's value by adding black.) If you can't determine the value, use your grey scale value checker. Squint down at your target color and note where it seems to visually
melt into its equivalent on the grey scale. Do the same with your mixture and adjust accordingly.
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Step 3 Saturation
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Since our hue and value look correct, we should now consider saturation. How pure is our reddish purple as compared to what is really showing on the apple? Sometimes we make the mistake of judging colors to be purer than they really appear. Decreasing saturation is sort of like neutralizing or greying a color but not completely eliminating its influence.
To change the saturation we can either add grey to a mixture, or add the mixture's complement. If we decide to add grey we need to make sure the grey is close to the same value as our current mixture. When adding grey we are in effect weakening its visual potency by diluting our mixture. Another way to reduce saturation is to add the color's complement. This is sort of like negating a fraction of the particles in our mixture to grey them out. If you add too much of the complement you completely grey the mixture. When adding a complement we might also have to go back and lighten the value as adding the complement can potentially darken the mixture (depending on the complement.)
In the case of the hypothetical now purplish apple mixture, we decide our mixture is a little too pure and so we decide we want to reduce the saturation. We add just a little yellow (the complement of purple) to neutralize it.
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Note, If desired, you can adjust the saturation first before changing the value. It's also important to note that changing the value can automatically adjust the saturation, since adding white or yellow or black to a mixture is also in effect diluting or de-purifying that mixture.
Hope I'm not making this sound too complicated.
In a nutshell:
Step 1 (Hue)
A.Choose a tube color that most closely approximates your target color.
If you don't have a tube of the desired hue, or if you are deliberately working with a limited palette, then mix the needed hue. (i.e. Yellow +Blue= Green. Red +Blue= Purple.Yellow +Red= Orange.)
B. Alter that hue if necessary.
(i.e. My tube of ultramarine isn't greenish enough to match this hypothetical bluish green cup I'm trying to paint, so I'll add just a dab of yellow to the ultramarine to green it up.
Step 2 (Value)
Once you approximate the generic hue, correct the value if necessary.
Add black or a complement to darken the mixture's value. Add white or yellow or both to lighten it. If you don't like using tube blacks mix your own black. (I.e. Burnt umber + ultramarine.)
Step 3. (Saturation)
Neutralize the colors purity if necessary by adding grey or a complement.
If you darken the value while neutralizing with a complement, refer back to step 2 and add a little white, yellow, or both to lighten its value.
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Important: When mixing a color, after each step, paint out a swatch of your mixture on a scrap piece of paper and then physically hold the swatch next to your target color and make a side by side comparison to check your mixed color against the reality of the target color.
Modify your pile on the palette as necessary - change the hue? value? saturation? Repaint your swatch, then compare again. No need to guess, or arrive at a needed color by accident. Be like a scientist objectively comparing your results with your target color. If you are struggling be assured it gets easier.
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Sometimes you'll find you can skip steps. The value of the tube hue might be fine but the saturation might be off. Or the hue and value adjustment might bring you to the correct target color with no need to worry about saturation. The best way to internalize this information is to experience it in practice for yourself. We painters need to be like paint mixing machines, mechanically and intuitively capable of mixing any color put forth before us. Our computer printers can approximate this with just 3 primary ink cartridges plus black.
Browns: For mixing purposes it might help to think of brown as a low value orange.
Black: Ultramarine plus burnt umber is one way to mix black. Feel free to use tube blacks instead. Black with just a little yellow added makes a great, earthy green.
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A Suggested Limited Palette for Acrylics:
To be able to mix a wider range of colors with just primaries, it makes sense to have an expanded set of primaries. Some suggest using a warm and a cool version of each hue.
For the blues, I'll use french ultramarine and cerulean. Reds: Alizarin Crimson and Cadmium Red. Yellows: Cadmium yellow (or y. ochre) and lemon yellow .
You also need white (titanium white and/or acrylic gesso), burnt umber, and black. You can mix your own black (i.e. ultramarine plus burnt umber) or just use a tube black.
Why use two versions of each primary? Limiting yourself to just one tube of each primary has limitations. If you want to mix ,for example, a turquoise, a cooler french ultramarine and cadmium yellow just won't work. Cerulean is closer to the turquoise and a more logical place to start. Mixing a bright vibrant orange just isn't possible with alizarin crimson. A warmer cadmium red would be in order. You don't have to lay out all 6 primaries for every painting, but choose your primaries as your subject dictates.
(Note: I originally prepared this information when I hosted a color mixing workshop on WetCanvas.)
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