|Urban sketching with my DIY Altoids mini kit|
The colors you use for this minimum palette can be any fitting a certain criteria:
Choose a warm and a cool version of each primary color.
Choose colors that would allow for mixing dark tones.
Choose colors that will be pure and bright when used alone.
These choices result in the widest possible range of color mixing results using the minimum number of pigments.
After choice of primary hues, the next consideration is pigment properties. Watercolor pigments can be transparent, granulating, opaque, sedimentary, or staining. Choosing a set of more transparent colors helps to avoid muddy looking color mixes. If your style requires lifting off of pigment to achieve light values, then you would probably avoid the staining pigments. If you like high tonal contrast (strong light and dark relationships) you would choose colors that are already inherently dark. They can be thinned out with water to create paler tones. If you like the textural qualities of your subject, then granulating or sedimentary pigments will enhance that effect.
The location in which you plan to sketch or paint will help determine which unique and particular colors you might like to include. A tropical location, for instance, would require a slightly different set of primary colors than one in a more northerly location. Seasonal changes or subject matter may require switching out one or two convenience colors. It depends upon what you are painting; landscapes, still life, florals, portraits, etc. Most of the time, however, you can get by with the dual primary palette, plus one or two convenience colors.
What are warm and cool primary colors?
A primary triad of red, yellow and blue can be split into a warm and cool version of each.
You need two yellows, one that is "lemony" and leans toward green and one that is "sunny", that leans toward orange.
Reds could be described as somewhat purple, leaning towards blue (Alizarin Crimson) and somewhat orange (Organic Vermilion) leaning towards yellow.
A blue that leans toward blue violet (French Ultramarine) and a blue that leans toward turquoise (Phthalocyanine Blue, Green Shade) will give you a wide range of mixing possibilities.
My Van Gogh (12 color) Pocket Box is shown above in action with a few custom colors switched out to suit my style. http://handprint.com/HP/WCL/water.html has a comprehensive section on watercolor pigments, artists' chosen color palettes with examples of their paintings, and much, much more!