Although Newton (1672) applied the term "primary" to colours with the meaning of "monochromatic", the expression as it is understood today has its origin in the historical concept that yellow, red and blue, initially alongside white and black, were the "simple", "primitive" or "primary" colours from which all others could be derived. The first visual representation of this concept is in a diagram in the Opticorum Libri Sex (1613) of Francois D'Aguilon (Figure 7.3) . D'Aguilon showed his "simple" colours on a linear scale comprising white, yellow, red, blue, and black respectively, and placed each of the three "composite" colours purple, green and gold  ("aureus") in relation to the relevant pair of simple colours, in an arrangement derived from music theory. From its mention in the commentary on the Timaeus of Plato by Chalcidius, the linear scale of simple colours is known to date back (alongside many other systems) to the fourth/fifth century CE (Kuehni, 2003). The system was repeated and elaborated in diagrams by other seventeenth century writers, and seems to have become increasingly widely held throughout the century.

The word "primary" was introduced by Boyle (1664), who reported:

"...I tell you, that the mixing of pigments being no inconsiderable part of the painters art, it may seem an encroachment in me to meddle with it. But I think I may easily be excused (though I do not altogether pass it by) if I restrain myself to the making of a transient mention of some few of their practices about this matter; and that only so far forth, as may warrant me to observe to you, that there are but few simple and primary colours (if I may so call them) from whose various compositions all the rest do as it were result. For though painters can imitate the hues (though not always the splendor) of those almost numberless, differing colours that are to be met with in the works of nature, and of art, I have not yet found, that to exhibit this strange variety they need employ any more than white, and black, and red, and blue, and yellow; these five, variously compounded, and (if I may so speak) decompounded, being sufficient to exhibit a variety and number of colours, such as those that are altogether strangers to the painters pallets can hardly imagine."

Boyle's statement shows an awareness of the concept of a gamut: while the primary colours suffice to mix colours of a full range of hues, some colours will, by their greater "splendor" (we would say chroma), lie outside this gamut. In reality any set of pigments produce a restricted gamut or range of colour mixtures, and are unable to mix colours outside this gamut. To accurately reflect this aspect of pigment mixing, the concept of primary colours needs to be formulated with this awareness, as a set of pigment colours whose mixtures enclose an optimal gamut of high chroma colours.

The step of removing white and bla