Specifying colour

The visible electromagnetic spectrum ranges between about 400 nm (violet) and 700 nm (red). In building, colours are rarely pure spectral colours (like the yellow from low pressure sodium vapour lamps), so defining them just by a single wavelength (589 nm in this case), measured by spectrophotometer, isn’t possible.

Colour physics

Real colours have hue (the colours of the spectrum, including the additive primaries red, green and blue, and the subtractive primaries yellow, magenta and cyan), chroma (the amount of colour), and lightness (closeness to white or black, the achromatic or neutral extremes). Different colour systems use various terms for the same concepts. All real colours can be characterized in this way, even fluorescents, though not all colour systems can cover them all.

Perception varies with the individual, the viewing conditions (e.g. illuminant, background, direction of illumination), and the reflectance of the material, and is non-linear. That is, the eye can detect finer than, say, 5 nm intervals in some parts of the spectrum, but not in others. In some ways this is parallel to sound perception.

Colour spaces

The three dimensions of hue, chroma and lightness are used to generate ‘colour spaces’. There have been many attempts to create colour spaces over the centuries, not all relevant to the reflective surfaces which are of most interest in construction. Some are very simple, and some are extremely complex (e.g. the number of hues varies considerably, some are purely subjective and others are based on colorimetry, some locate complementary colours opposite each other on the colour circle, some have a simple overall geometry and some are freeform). See www.colorsystem.com  for an historical survey. Generally, chroma ranges are at their maximum halfway along the lightness dimension, so most colour spaces approximate double cones.

The overarching colour space that most colour systems correlate to in varying degrees is the CIELAB (or CIEL*a*b*) colour space, promulgated by the International Commission on Illumination (CIE) (www.cie.co.at ). It is defined in CIE 15: 2004 Colorimetry, and was originally published in 1976. The CIELAB colour space has lightness/darkness along the vertical axis (L*), redness/greenness along one horizontal axis (a*), and blueness/yellowness along the other (b*). The CIELAB spectrum ranges from 420 to 680 nm. Standard CIE illuminants and observers (2° and 10°) are defined in joint ISO/CIE standards, in effect creating multiple CIELAB colour spaces.

Colour atlases

Colour atlases come in two types – those defined only by published standardized swatches, and those defined by colorimetry, with swatches as the best approximations technically possible for a selection of possible colours. The second type can be seen as implementations of the CIELAB colour space, or some variant of it. There is no official CIELAB colour atlas.

Colours defined only by swatch

Mapping between swatch-based systems can be approximate at best – exact colour matches in the limited ranges offered are unlikely. Some colours may have no near equivalents in other systems. The systems are not extensible by the user, though their publishers extend them from time to time.

BS 5252 and BS 4800

BS 5252:1976 Framework for colour co-ordination for building purposes, establishes 237 colours (a colour-matching fan is available for this standard). Derivatives from BS 5252 include the well-known BS 4800:1989 Schedule for paint colours for building purposes, which covers 100 colours (including black and white). Others include BS 4901:1976 for plastics, and BS 4904:1978 for external cladding.

Colours are identified using a 3-part notation, describing hue, greyness and weight, e.g. 08 B 15, a yellow-red. There are 12 hues plus a neutral (00). Greyness has 5 steps, A to E. Weight, an equivalent concept to nuance in NCS (see below), subjectively combines greyness and lightness and ranges from 01 to 56. Lightness is part of the system but is not included in the notation. The standards include an approximate mapping to the Munsell system (see below).

RAL Classic

Another colour system commonly invoked is the German RAL Classic ( www.ral.de/en/ral_farben/home/index.php ). This system defines 208 colours, with four-digit notations and standardized names in five languages. The notations run from yellows (1000s), oranges (2000s), reds (3000s), purples (4000s), blues (5000s), greens (6000s), to greys (7000s), browns (8000s), and whites, blacks and metallics (9000s). Examples include RAL 5013 Cobalt blue, and RAL 7023 Concrete grey. The base standards are RAL 840-HR for matt colours, and RAL 841-GL for gloss colours. The system is available for various CAD packages, through RAL Digital.

RAL is a German standardization institute that also deals with eco-labelling.

Pantone

The Pantone system ( www.pantone.co.uk ) began life in 1963 in the USA, for defining colours for printers, but expanded into other fields later, e.g. textiles in 1984, plastics in 1993, and architecture and interiors in 2002. The latter system covers 1925 colours, each of which has a 6-digit numerical notation (e.g. # 19-1764) and an ‘inspirational’ colour name. The colours are grouped by hue, in ‘colour families’. The system is available for various CAD packages.

Colours defined by colorimetry

Accurate mapping between colorimetry systems, and to the CIELAB colour space, is possible. Users can also ‘invent’ new colours if this was felt to be necessary – though suppliers may not be able to deliver due to limits in colourant technology.

See www.easyrgb.com  for free online conversion software, including to some proprietary colour ranges such as ICI’s. Munsell Conversion Freeware is available from the GretagMacbeth website ( www.gretagmacbeth.com ).

Munsell color order system

Albert Munsell first published a colour system in 1905. The current system is marketed through GretagMacbeth in North America, which is where it is mostly used, and D.G. Colour Ltd ( www.dgcolour.com ) in the UK and elsewhere. The primary source for the standard colour swatches is the Munsell book of color, which describes over 1600 colours (gloss) in 40 hues.

The Munsell colour space is similar to CIELAB’s but, as with other systems, notation differs. Munsell’s is based on hue, value and chroma – H V/C for chromatics, and N V/ for achromatics. The Munsell hue circle has 100 hues in 10 hue sectors ranged around the colour circle: R, YR, Y, GY, G, BG, B, PB, P and RP. BG corresponds to cyan and RP to magenta. Indeed there is an alternative coding, using C and M etc, called the ‘primary hue circle’. Hue notations range from 1R to 10RP – just as there is no 0 o’clock, so there is no 0R. Value runs from 0 (pure black) to 10 (pure white), and chroma runs from 0 (neutral) to over 20 for normal reflecting colours, even up to 30 (fluorescents). Maximum chroma varies with hue – yellows can have higher chroma than blues, for example.

Examples are 5.3R 6.1/14.4, for a red, and N 5/ for a grey neutral.

The Munsell system has been implemented in several national standards, including ASTM D1535-01 Standard practice for specifying color by the Munsell system (includes mapping to the CIELAB system), and JIS Z 8721:1993 Colour specification – Specification according to their three attributes. BS 5252 and DIN 6164 also pick it up.

DIN 6164-2

DIN 6164-2:1980 DIN colour chart; specification of colour samples, is one of several DIN standards aligned to CIE practices. This one defines around 600 colours, characterised by hue, saturation and darkness (T:S:D), e.g. 22,5:3,2:1,7 (commas are decimal points – get used to it!). There are 24 hues (around the colour circle), up to 6 saturation steps (0 is achromatic), and up to 7 darkness steps.

RAL Design

Based on colorimetry to fix the self-confessed ‘cracks’ in the RAL Classic system, this system corresponds to the CIELAB colour space. It comprises 1688 colours, each with a seven-digit notation describing hue, lightness and chroma, e.g. RAL 210 60 30. Hue, the horizontal angle in the colour space, runs from 010 to 360 – in 10° increments, so there are 36 hues. Lightness, the vertical axis of the colour space, runs from 0 (black) to 100 (white). Chroma corresponds to the distance from the vertical axis, with achromatic colours at zero. Saturated (maximum chroma) colours vary from hue to hue and with lightness so, as in the CIELAB and Munsell colour spaces, the envelope is an irregular shape. The system is available for various CAD packages, through RAL Digital.

Scandinavian Colour Institute Natural Colour System (NCS)

The NCS colour space (www.ncscolour.com ) is constrained to a double cone, with black and white (S, W) on the apexes, and the four elementary colours (Y, R, B, G) at the cardinal points around the rim – so the system is akin to CIELAB. Vertical sections through the space take the form of colour triangles, one for each hue, in which colours of the same nuance, whiteness, blackness, chromaticness, saturation or lightness can be readily identified.

The NCS ATLAS 1950 Original (2004) covers 1950 colours. Notation is by nuance (blackness and chroma), and hue, e.g. S 1050 – Y60R. Here, S indicates that the colour sample is part of the NCS ATLAS. The 10 is the percent of blackness, the 50 of ‘chromaticness’ (so whiteness = 100 – 10 – 50 = 40). Y60R indicates a yellow with 60% redness and 40% yellowness. Achromatics have no hue so are only given nuance values, with chromaticness of 00, and fall in the range 0300-N (white) to 9000-N (black). Mappings to 200 RAL colours, BS 5252 and Munsell are available. The system is available for various CAD packages.

Conclusion

No wonder many architects prefer white! But even this isn’t as straightforward as it sounds. A single shade of white can be hard to pin down, as Richard Meier knows, from his Museum für Kunsthandwerk, Frankfurt am Main, where the same shade of white was required across a wide range of materials and finishes. You need to be able to communicate precisely which white you want. And these systems are the way to do it.

Colour system overview