Split primaries model

The split primary colours model is a practical solution to the limitations of the RYB model, namely its difficulty to produce all-round satisfying secondary colours from a limited set of three primary red, yellow, and blue colours.

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

The split primary colour model categorizes primary colours as being biased towards one of its adjacent secondary colours. Thus, reds are either orange-reds or violet-reds, blues are either violet-blues or green-blues, and yellows are either green-yellows or orange-yellows. This categorization allows for mixing a richer variety of hues with a rather limited palette of six colours (two set of primary colours). It also allows for a good degree of prediction when mixing secondary colours.

In order to mix bright secondaries, the artist should choose primary colours biased towards the same secondary. For example, an orange-yellow and an orange-red would mix brighter oranges than any other red-yellow mixes. Consequently, in order to mix dull secondaries, the artist should choose primary colours biased towards opposite secondaries. For example, the dullest orange can be obtained by mixing a green-yellow and a violet-red. Finally, middle of the range oranges can be obtained by mixing orange-yellow and violet-red, and orange-red and green-yellow.

The split primary colour model is quite intuitive, as it works on the RYB model assumptions for colour mixing but increasing the level of usable hues. It also allows for a good deal of prediction and control regarding the secondary hue to mix, be this brighter or dull secondaries, or somewhere in between.

One of the limitations of this theory is that it assumes that it is not possible to have middle of the range primary colours but that all colours are imperfect and thus, biased to one of two secondaries. Consequently, no set of three primaries will produce a satisfactory, all-round, gamut of colours. That is, it cannot accept modern colour theory based on the subtractive primaries cyan, magenta, and yellow, which has been proved to be able to do so in CMYK printing processes.

Although above limitation should render the split primary model model obsolete, in practice, a well-selected split primary palette will produce a wider colour gamut than a CMY palette (or RYB palette), simply because it widens the initial colour space beyond that of CMY. Yet, this can also be achieved, perhaps more satisfactorily, with an "hexachrome" palette or a secondary palette.

Want to know more?

Handprint - Split "primary" palette
Bruce MacEvoy discusses the split primary palette further in this page. It may be a bit difficult to follow at times, but it offers a basic description and criticism to the palette and, by extension, to the underlying model.
Wikipedia - Color theory
Wikipedia has a small section on split primary colours in this page.