The type of dark pigment you use doesn't really matter. Vine, ivory or lamp black work equally well, though they may handle differently depending on manufacturer. Natural sepia was the traditional (18th century) color for building the value structure of a watercolor painting or ink wash drawing. Modern convenience mixtures, labelled sepia, neutral tint, payne's gray or indigo, are intriguing because they seem to change hue as they are diluted toward tints, creating a beautiful duotone effect. Any of these dark shades gives a nearly complete value range.

I have tried value paintings with a synthetic black that is mixed from red, green and blue violet pigments. Slightly changing the proportion of each pigment in the mixture allows the painter to shift around the hue bias, but as three paints are involved and none of them are "primary" colors, this method resembles the kind of color mixtures one gets from a Velázquez palette.

The monochrome palette is not just for students and sketchbooks. By eliminating color, and using a single dark paint with little or no pigment texture, the artist must focus on two key design elements: value structure, and brushwork (brushed texture). The palette puts the focus on the value composition of a painting, and emphasizes problems of light and atmosphere. And without color to distinguish among objects, brush technique becomes much more important. Precise edges, perfectly graded shadows, diverse brush marks (including drybrush), and the texturing effects of blooms, blotting, washes and resists are all displayed more dramatically.

The illustration painting, an early work by Paul Klee, is typical of his watercolors in the period 1908-11, when he painted almost exclusively with only black paint (occasionally adding white gouache for highlights).

A monochrome palette is a wonderful discipline when used with a book of black & white photographs — among my favorites are Adams's landscapes, Mappelthorpe's flowers, Weston's nudes, Penn's or Man Ray's portraits and still lifes, and Strand's abstractions. Copying (or reinterpreting) these images sets you thinking about value composition directly. The palette also provides plenty of exercise in the brush skills necessary to model subtle changes in light across rounded surfaces or diffuse shadow edges, or to show the effects of air and humidity through value alone. Cloudy skyscapes are probably the ultimate technical achievement with this palette.

Because of their low specific gravity, tiny particle size and extra helping of dispersant, carbon pigments are among the most difficult to control in watercolor paints. Active wet in wet, they diffuse and backrun readily, and tend to leave tiny, bright pinholes in wash areas that were solid color when wet. They also lighten significantly as they dry, but in proportion to their value when wet: tints shift very little while the darkest color shifts a lot, and the painter must anticipate drying shifts by applying dark values darker than they should appear in the finished painting.

Finally, carbon pigments discourage fussing. They are very nearly impossible to rewet and lift, frustrating any attempt to lighten areas that are too dark or to edit messy edges or accidental drips. A second or third layer of diluted color can be used to darken areas that dry too light, but this is a crutch that gets tedious very quickly. Klee habitually used partly overlapping layers of paint to create geometrically patterned value and color gradations in his watercolors, but each of his layers is exactly judged — a skill he learned by using a monochrome palette.

It's surprisingly difficult to reproduce a black and white photograph with a single pigment. Much of the effect of a photo arises from slight differences in the mid values, and many painters lazily lean on hue contrast to increase the apparent differences among the midvalue areas of a painting. Without hues to distract you or help you, accurate values must be painted directly. If a black and white original isn't challenge enough, try painting from a color photo. Even without hue, the warm/cool contrast can be expressed by using lighter values to represent warm surfaces and darker values to express cool; the ancient value ranking is white, yellow, orange, red, green, purple, blue and black.