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the art of formulating colour

Architect's memo 68: November 2001

Of all the aspects of paint technology that we paint chemists consider absolutely vital to a truly fulfilled life, it is really only colour that has universal appeal.

It is illuminating (sorry) to realise that all colour derives from and is contained within a narrow band of light waves. Without light there is no colour.

Colour also needs a detector that can distinguish between mixed packets of light waves and we humans have been blessed with one of the best. The two million rods and cones in the human eye can detect up to ten million overlaid shades of colours.

Televisions produce colour by blending coloured light sources in a process called 'additive' colour matching. Paints however, rely on coloured pigments that work by absorbing (or taking away) parts of the light spectrum and only emitting that part of the spectrum by which we characterise colour. For example, ultra-marine absorbs the yellow and red part of the spectrum and only emits blue, which our eye then recognises. Every additional coloured pigment or tinter that is added to a base paint will take away light, and hence this method is known as 'subtractive' colour matching.

Almost everyone knows that it is almost impossible to brighten a dull colour, but very easy to dull a bright colour. Because of this, the very cleanest or purest colours are achieved by using a single chromophore. The addition of several chromophores can give very interesting and subtle colours, but always duller than the single chromophore.

(We have used the term chromophore here to denote single 'packets' of colour rather than pigment, which can already contain 'mixed packets' of colour).

The mixing of complementary colours will produce black and indeed Monet was known for never using a black pigment, preferring to produce his blacks by the blending of complementary colours. The fact is, however, that black is black however one produces it. Black is the total absence of reflected light, whether it is achieved by complex colour mixing or by simply using soot.

Every colour can be assigned a spectral reflectance curve to uniquely fingerprint that shade and there is no aesthetic 'true' or 'real' way to reproduce that shade. No matter what combination of pigments is used to accurately reproduce that spectral reflectance curve, the colours will appear identical under a given illuminant.

As we said earlier, subtle colours can be created by the mixing of complex pigment combinations and indeed, it was Monet's 'not quite blacks' which gave quality to his work. Complex combinations have also been used, especially in the OEM world of automotive finishes, in an attempt to give a 'hard to match' uniqueness to products.

The downside is that such colours can be very hard to reproduce and it is not unusual to have up to six variants of one automotive colour, which keeps the body shops on their toes.

K.I.S.S. is a very good rule in many areas and especially so in formulating colour. Tinters can have a detrimental impact on the durability of the paint to which they are added, thus the paint chemist seeks to develop colours that require the minimum number and amount of tinters. The perceived benefit of additional subtlety (which may be visible to someone with a highly sensitive eye) may not be worth trading-off robustness, reliability and reproducibility.

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The Resene architect's memo section provides technical information on a variety of topics relating to paints, finishes and coatings.

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