Light and Pigment Compared
Painting technique is a dauntingly large and rather vague subject to discuss. We could provisionally define 'technique' as the skills involved in accomplishing a specific artistic intent. But this leaves the notion of 'artistic intent' unexamined (it obviously takes infinitely various forms among artists) and tends to suggest that such an intent is rather imposed on the medium of paint. There is no one prescriptive model of 'good technique'. One can, however, distinguish a narrower, more practical sense of the term as the knowledge consisting of certain axioms of advice on how to avoid using paint in ways that will produce impermanent or visually ineffective results.
When artistic instruction was centered on the workshop, questions of aesthetics and soundness of physical procedures were inseparable. But towards the end of the 17th century, workshop training began to be supplanted by that of the Academies, whose curricula were generally confined to drawing in dry media. This 'de-skilling' of the business of applying paint caused damage to artistic formation and was the object of frequent criticism.
Oil paint is a suspension in one of several kinds of drying oil, either of a coloured dry pigment or of a coloured liquid or 'lake' with an inert powder added to give it body. In both instances, we are dealing with coloured substances which seem to possess a particular colour because each absorbs to a varying extent some parts of that spectrum of coloured light we perceive, in total, as 'colourless' white light whilst reflecting, to a varying extent, other parts.
Newton famously demonstrated that daylight was not one but virtually the whole range of all 'pure' colours visible to us by breaking it down with a glass prism. Subsequent physicists have concluded that the range of these 'spectrum' colours (red-orange-yellow-green-blue-violet) is, subatomically, a range of frequencies with which the packets of energy we call photons hit the human retina. The red end is the lowest frequency visible to the human eye; the violet end is the highest.
The spectrum is not several distinct frequencies but a continuous spread of them. But suppose we select a typical or median point from each of the three general 'zones' of these frequencies, one from the red-orange zone, one from the yellow-green, and one from the blue-violet. In that case, we can construct a triad of coloured lights from which, if intermixed, most of the remainder of the spectrum can be reproduced, and if the triad is thoroughly intermixed (known as additive mixing), the result is white light. This triad of coloured lights is generally known as the triad of primary additive colours.
The behaviour of coloured light is fundamental to colour theory. Still, the colours of the light resulting from the mixing of adjacent primaries of coloured lights suffice to demonstrate to anyone who has used a palette that the behaviour of coloured substances like paint is usually quite different. This is because the colour we perceive a given paint to 'possess' results from the interaction of light that is generally full-spectrum, with a surface that absorbs some parts of that spectrum and, to a greater or lesser extent, reflects others.
In fact, the portion of the spectrum reflected by most coloured substances is far broader than we might in common sense judge them to have as their actual colour. We are psychologically predisposed to conceptualise and distinguish colours in accordance with 'colour constancy', which leads us to assume that the part of the light spectrum which a given substance reflects most of all is its 'proper colour'. If we did not have such a tendency, we would find the day-to-day recognition of objects much more difficult.
But there are many instances where the full spectrum of standard daylight is not present, for example, in a room which has intensely coloured window glass or walls. In these instances, that part of the spectrum of light usually reflected by a given substance might be only weakly present in the light falling into and reflected by the room, and our perception of the colour of that substance changes accordingly. This is the cause of that well-known irritation to clothes-buyers and artists alike: metamerism - the change in the colour of objects determined by change in the colour of the light falling on them.
The complexity of the reflectance of light by paint surfaces explains why the range of their colours far exceeds those in the light spectrum alone. Brown and red earth colours are substances which only weakly reflect certain wavelengths of the red-orange portion of the spectrum whilst absorbing almost completely the yellow-to-violet remainder. This complexity also explains the richness of paint colour in manipulation. The pattern of reflectance of light across the spectrum by each colour is highly idiosyncratic and can be plotted in graph form.
For example, the reflectance profile of a given red pigment peaks at a certain point in the red zone of the spectrum but only tapers away gently on that portion of the graph covering the adjacent orange zone, whilst the profile of another red peaks at virtually the same axial point but tapers sharply away through the orange zone and then tapers only gently through the green-blue-violet zones. In dry or opaque form, these two might appear indistinguishable. But when these pigments are made into paint and brushed out, their differences are immediately apparent: thinner and more transparent zones will disclose the ancillary 'taper' or 'undertone' of colour.
The traditional artist's primaries were an unwitting attempt to make a good selection from the spectrum in a manner anticipating those of physicists. The broad reflectance of the physical primaries means that each absorbs only about one-third of the spectrum: blue reflects a fair amount of green and absorbs only red; red also reflects blue whilst absorbing green; and yellow reflects green whilst absorbing blue. In an approximate way, these artistic primaries' reflectances overlapped sufficiently to 'cover' the spectrum.
The usefulness of any physical colour primary triad, whether of printing ink or paint, lies in its forming a basis for mixing colours that we can judge to be different, and consequently it will be shifted to that side of the light spectrum to which we are most cognitively sensitive.