Secondary palette

The secondary palette is a colour model derived from the hexachrome model, but with black substituted by a violet blue. The initial six hues are thus, yellow, green, cyan, violet-blue, magenta, and orange.

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(Image embedded from Handprint.com on 2 January 2011)

The main difference with the hexachrome model is, thus, the use of a violet-blue instead of black in order to mix brighter violets and produce deep tones. This is probably so because of the tendency to avoid using an opaque black in watercolours (and painting, in general), while also extending the range of possible violets to be mixed from blue and magenta (a mixed violet may be brighter than a single pigment violet such a dioxazine purple).

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Handprint - Secondary palette
Bruce MacEvoy discusses the secondary palette in this page.