As with value and hue, the best way to identify chroma is in terms of relationships. How intense is the color you’re looking at compared with the intensity of other colors around it? Chroma can be hard to separate out from value; light colors sometimes look more intense than they are, and dark colors sometimes look less intense. You get better with practice.
Enough with those bright orange skin tones already!
Working with low-chroma color
Almost all paints are high in chroma right out of the tube. Since most of the world is low in chroma, the majority of a realistic painting will consist of neutrals and near-neutrals. So a realist painter is going to have to spend a lot of mixing time reducing the chroma of paint.
Reducing chroma with mixing complements
Other than yellow and violet, it is very helpful to experiment with, and memorize, pairs of complementary colors. As a general rule, if you want to dull down an intense color, choose a dull complement. Blues have mixing complements in the range of warm yellows, oranges, and middle reds. Middle and cool greens have mixing complements in the range from middle reds to violets. Warm greens have mixing complements among the violets. Some of my favorite mixing complements include raw sienna/ultramarine blue, viridian/pyrol ruby, Prussian blue/Venetian red, and ultramarine blue/raw umber. I expect that most artists develop a set of strongly preferred mixing complements.
Reducing chroma with optical color mixing
Reducing chroma with white
Reducing chroma with grey
Reducing chroma with glazing
If you paint one color thinly over another color, you get an optical mixture. Blue glazed over yellow produces a green, for example. You can use this effect to reduce chroma, since an optical mixture is darker and duller than the colors that make it up. Michelangelo, for example, sometimes made a dark dull blue by glazing ultramarine over black.
The browns and the brown-ish
Working with high-chroma color
Because of the way the visual system works, a color is perceived as more chromatic when it is placed next to its visual complement. The impressionists made frequent use of this principle. A bright magenta looks more intense when it is surrounded by a dull green. The complements identified in the Munsell color system more accurately reflect human color vision than those in the antiquated three primary color wheel.
Masstone and undertone
Maintaining chroma at high values
Maintaining chroma by glazing
One effective way to maintain chroma is by glazing. If you apply paint very thinly over white, you can get a higher chroma than you could by mixing that paint to the same value using white. And a transparent paint that is applied a little more thickly can be more chromatic at low values than you might be able to obtain with a mixture of the same hue.