4 : nickel dioxine yellow (PY153), quinacridone rose (PV19), phthalocyanine blue GS (PB15:3), phthalocyanine green YS (PG36) In the history of "primary" colors, the Renaissance artist's "basic colors" (as described by Alberti or Leonardo) were not three but four red, yellow, green and blue.
This selection was suppressed during the 18th century in favor of three paints, a "color theory" prejudice that continues to this day. But 20th century color researchers have established that there are two fundamental contrasts or opponent processes in color perception, and that these are approximately the four unique hues or Hering colors. Thus, vision science affirms the renaissance idea that four primaries the artists' "primaries" can be a useful framework for color design and color mixing.
Many variations on the basic four color scheme are possible. To work out the paint choices for this palette, use the same color logic illustrated for the "primary" triad palette: seek harmonious fundamental colors, careful balancing of saturation costs, ease in color mixtures, and the ability to mix rich darks. Now you choose four paints to meet those goals:
Yellow. Yellow is a good first choice to make, as it helps to determine the location of the other hues. I most often choose a "middle" yellow, such as benzimidazolone yellow (PY154) or hansa yellow (PY97), rather than a green ("light" or lemon) yellow such as hansa yellow light (PY3) or cadmium lemon, to keep the warm color mixtures (mixed yellow and red) vibrant. To increase the warmth of the palette even further, you can choose an orange yellow, such as hansa yellow deep (PY65) or cadmium yellow deep. I highly recommend either nickel azomethine yellow (PY150) or nickel dioxine yellow (PY153) for their duotone hue shifts, transparency and good tinting strength. Neutralized by mixture with the other paints on the palette, a warm yellow in particular can effectively imitate a variety of earth pigments (dull yellows, browns, reds), yet still mix beautifully natural greens. (The demonstration painting was made with nickel dioxine yellow.)
Red. In a minimal palette it is possible to choose a cool or violet red pigment without affecting the saturation of mixed warm colors (see this example). Quinacridone rose (PV19) is a relatively lightfast, transparent and versatile substitute for alizarin crimson (PR83) or rose madder genuine (NR9) both fugitive pigments you should avoid in any watercolor paint. Quinacridone rose holds a dark value in masstone, mixes saturated reds and oranges with most yellows or orange yellows, and mixes subdued violets and violet blues with phthalo blue. Other good alternatives include quinacridone magenta (PR122, excellent with a warm yellow) and, for subdued red orange mixtures that can reach to lovely dark values, quinacridone violet (PV19). If you have chosen a cool yellow, you may want to experiment with a warmer red, such as quinacridone pyrrolidone (quinacridone carmine, PR N/A) a red any warmer (closer to orange) than this will yield violet and violet blue mixtures that you may find too dark and dull, and warm mixtures that may have too much of a yellow bias.
Blue. Phthalocyanine blue (PB15) is often chosen for its relatively saturated green mixtures, which is acceptable in the demonstration palette because the blue is paired with a violet red (if the red were closer to orange, it would mix disappointingly dull violets with phthalo blue). Choose a phthalo blue that is not too green, however, to keep the violet mixtures from getting too dull. Phthalo blue has a nice liquid texture in washes, and mixtures with quinacridone rose will make very acceptable replacements for violet and blue violet paints. Among these, cobalt blue (PB28) or ultramarine blue (PB29) are interesting alternative blues for a four paint palette: they will mix brighter violets with quinacridone rose or quinacridone magenta, and lend beautiful textural effects to washes and mixed colors.
Green. Phthalocyanine green BS (PG7) gives the most saturated turquoise and green blues mixed with phthalo blue, and mixed with ultramarine blue creates an effective substitute for cerulean blue (PB35) or cobalt turquoise (PB36). This is a dark valued but fairly saturated green, so the yellow greens it mixes with almost any yellow will be adequately intense, although dark; you will probably want to mute these mixtures with a touch of your red paint to make the greens more natural. Phthalo green YS (PG36) is lighter and yellower hue, closer to a balanced green, and produces brighter green mixtures with yellow. Powerful darks and neutrals are possible with either phthalo green in mixtures with quinacridone paints. Other good green paints for this minimal palette are convenience mixtures such as hooker's green or sap green, or granulating and duller paints such as viridian (PG18) or cobalt green (PG19).
Your choice of four paints is usually aimed at keeping the warm mixtures acceptably saturated, placing most of the saturation costs in the purple mixtures, and providing a dark mixture (for example, using quinacridone carmine and phthalo green) that at full strength is close to black. If chosen well, these four paints can replace all other paints from your palette. But all four paints must work well together.
The main drawback to the recommended palette is that the saturation of mixed colors around the violet and warm side of the color wheel is about the same as it is in the "primary" triad palette. The saturation of mixtures on the warm side of the wheel could be improved by closing the distance between the red and yellow colors, for example by choosing a warmer red or a warmer yellow (as I did in the demonstration painting, by using nickel dioxine yellow). This might require an adjustment to the blue paint selection to keep the mixed violets from appearing overly muddy. However, these tradeoffs illustrate that you can adjust saturation costs through the unequal spacing of colors around the color wheel.
Because all the recommended paints are synthetic organic pigments, the recommended palette won't yield granulation or texturing effects, which must be produced through varied brushstrokes and accenting watermarks such as backruns.
However, in most paint brands, these four paints will have similar transparency and tinting strength (the cool paints, because they are darker, will be somewhat stronger in mixtures). This will help you learn the proportional mixture or "color recipe" required to get any specific hue or saturation around the hue circle, especially for elusive colors such as tans, browns and olive greens. Once you learn these fundamental recipes, you will learn through experience to reproduce them by compensating for changes in the tinting strength or color of different paints.
The artists' "primaries" palette is probably the best way for a novice painter to learn color mixing in relation to the four major quadrants of the color wheel experience that is much harder to develop if the student jumps in with a palette of many colors.