11 : cadmium yellow lemon (PY37), aureolin (? PY40), quinacridone maroon (PR206), cadmium scarlet (PR108), pyrrole red (PR254), opera (? PR122+BV10), ultramarine blue (PB29), iron [antwerp] blue (PB27), cobalt blue (PB28), cerulean blue (PB35), payne's gray [hue] A glance at the arrangement of the reds, yellows and blues shows that Nita Engle's palette is a fairly conservative extension of the split "primary" palette.
A bright red, a dark "earth" color (quinacridone maroon PR206), a dark shade paint (payne's gray), and two additional blues are her additions to the basic split primary design. If we consider these in terms of the four fundamental palette limitations, then the major adjustments have been primarily in the value range, pigment attributes, and mixing convenience. The chroma is not expanded much by the two reds, violet mixtures are made significantly brighter by the addition of three blues; but green mixtures are kept subdued by the two greenish blue paints on the palette weakly tinting, light valued cerulean blue and the dull, dark iron blue.
The primary yellows (aureolin and cadmium lemon yellow) have similar hues aureolin is slightly warmer which implies they are split primarily to get a contrast them on handling characteristics. They differ in transparency, staining and mixing range.
There is less of a difference between the pyrrole red and cadmium red the cadmium is darker, more opaque, less saturated, and has a slightly smaller mixing range, giving substance to "earth" mixtures. A red comparable to the pyrrole red can be mixed from the opera and yellow paints, but Engle advocates a wet in wet tinting method that mixes iridescent atmospheres by sloshing together red, yellow and blue puddles. The pyrrole red is a convenience paint in that process.
The payne's gray is necessary to achieve deep darks, because there is no dark green as a mixing complement to the red or magenta paints. However, Engle prefers to paint fogs, oceans and snowscapes, and for these motifs the effects possible with diluted payne's gray can be highly evocative.
The blues serve to amplify the small hue differences between the yellow and red primary pairs. Ultramarine is warm, semitransparent, saturated and dark, while the cerulean is cool, semiopaque, unsaturated and pale. Cobalt blue is a semitransparent compromise between the two, while iron blue is moody and susceptible to diffuse or backrun. As I document in the guide to watercolor pigments, variants of iron blue labeled antwerp blue are typically less lightfast than the usual prussian blue (PB27), so this is a choice I don't recommend you repeat.
The peculiarity is Engle's use of Holbein's opera (listed under PR122), a very intense and marginally impermanent magenta paint with a good mixing gamut across the warm hues. Quinacridone magenta (PR122) is the main ingredient in opera and provides a similar hue and high saturation with better lightfastness.