Architects memo no.
68: November 2001
the art of formulating colour
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'
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'
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