Hexachrome model

The hexachrome palette, or hexachrome model, is a trademarked palette for colour printing which uses six hues (thus, hexachrome) in order to cover the widest colour gamut possible. Although "hexachrome" means six colours (thus, it could be equally applicable to the split primary model), its trademarking makes it exclusive to six particular colours: the four CMYK printing colours (cyan, magenta, yellow, and balck), and the two "secondary" colours, orange and green, all colours derived from pigments rather than by mixing.

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

This hexachrome palette allows for obtaining the brightest initial hues (eg, an "original" orange is brighter than any orange mixed from red and yellow) and, thus, the widest colour gamut possible with a minimum of pigments.

The hexachrome model is, perhaps, the most practical approach for colour printing, novice artists, and artists wishing to have a bright palette. In all these cases, the model offers the brightest hues possible, which can then be subdued or biased as appropriate (eg, if artists need a duller orange they can mix it easily from bright orange subdued with its complementary, blue). They can also quick start a painting project without the need to mix a hue before hand. Artist may also substitute the black with a violet.

However, if a bright palette is not in the artist's repertoire (eg the artist prefers a subdued palette), then the hexachrome model may not be the most efficient to use.

Want to know more?

Wikipedia - Watercolor painting
Wikipedia has a small section describing the hexachrome palette in this page (see here).
Wikipedia - Hexachrome
Here is a bit more of information on the hexachrome process, as trademarked by Pantone Inc.