14 : cadmium lemon (PY37), cadmium yellow (PY35), yellow ochre (PY43), cadmium yellow deep (PY35), cadmium red (PR108), alizarin crimson (? PR83), dioxazine violet (PV23), ultramarine blue (PB29), iron [prussian] blue (PB27), cobalt blue (PB28), cerulean blue (PB35), sap green [hue], ivory black (PBk9), chinese white (PW4) The Willis palette is a good example of an "extended" split "primary" palette, instructive because it shows how the mixing limitations of that palette can remain even after you add more paints.
The close doubling of the reds, yellows and blues is the signature of the split "primary" palette. Willis in fact chooses four yellow paints in total, three of them cadmium yellows, shunning a more evely spaced selection of paints (including an orange and scarlet) across the warm hue range. Both reds are rather dark.
The closely spaced yellows reorient the effective warm/cool contrast to run between yellow and middle blue. The duplication along this mixing line of two unsaturated colors yellow ochre and prussian blue suggests that the palette is implicitly shifted toward a complementary color scheme, with middle yellow and middle blue as the key complementary pair. (Note the balance of analogous blue and yellow hues around those two colors.) This is consistent with a definition of the warm/cool color contrast as lying between yellow and blue rather than scarlet and cyan.
Crossing this yellow/blue axis is a weaker red vs. green contrast: fewer paints are needed because the red green antagonism is so potent. The single moderately saturated and dark green, the convenience mixture sap green, also contains a substantial amount of "yellow" color, and it is approximately as dull as alizarin crimson (which is of course a fugitive pigment and a serious defect in a professional palette).
The cadmium red provides a saturated but relatively dark red, with results in relatively dark, dull scarlet and orange mixtures with cadmium yellow deep. Muted with black or ochre, these produce the handsome range of tawny colors on display in the demonstration painting. For this reason, all the "essential" earth colors (including the seemingly indispensible burnt sienna) are dispensed with. Instead, muted yellows, oranges or reds are mixed directly from the cadmium red and cadmium lemon yellow, darkened as needed with a touch of sap green or the marginally lightfast dioxazine violet.
These somewhat muted warm mixtures provide a better color balance with the wide selection of purple, blue violet, blue and cyan blue paints, and gives Willis a landscape light with a slightly cool or blue bias (because the oranges are dulled and the blues are brightened). This is characteristic of the idea of daylight as containing a bluish tint. This is visible in the demonstration painting as the relatively dull warm hues, the chromatic shadow purples, and the rather dull foliage sap green (which is clearly duller than the tube color), which would be dulled and darkened by predominantly blue illumination. The overall effect is of intense daylight illumination.
Willis's choice of blues ultramarine blue, iron blue, cobalt blue and cerulean blue is conservative. She avoids the phthalo blues, probably out of dislike of their high tinting strength and tendency to stain the paper, or because she fears they will "stain" or dull the cadmium paints. The selected blues provide a wide range of pigment textures, and in most brands of watercolors they lift easily. Cadmium red makes dull mixtures with all these blues, and alizarin crimson makes dull violets, so the rose to violet side of the color space is brightened by dioxazine violet (used in the shadows of the demonstration painting).
The emphasis on light also requires care in the handling of the value structure of lighted and shadowed areas, and for this reason Willis includes both a black and a white paint in her palette. Willis does not use scraping, lifting or resist (maskoid) to reserve her whites. Chinese white is used primarily to make corrections or insert highlights in the finished painting; the more opaque and lightfast titanium white, PW6 might be a cooler and more suitable choice. Ivory black is used to tone down colors and to provide a centered dark value; it can also create a lot of color variety when mixed with dark dioxazine violet, sap green, prussian blue, alizarin crimson and yellow ochre.
Willis handles these complexities well in the way she mixes and applies these paints, getting the value and texture correct on the first attempt (because overpainting or fussing will produce an opaque "mud"). This is a "master palette" that demands quite a lot of skill with mixing and brushwork to be effective. The lesson is that palettes are not just a selection of paints for color mixing, but also a selection of paints that works best with certain techniques for applying paint to paper.