Subtractive colour mixing

Colour technologists actually use three visual characteristics to describe the general characteristics of dyed or pigmented materials  

The accepted definitions of these characteristics, which will be used in this text, are those established by the Society of Dyers and Colourists  

Hue those attributes of colour whereby it is recognised as being predominantly red, green, blue, yellow, violet, brown etc. 

Strength (of a dye) the colour yield of a given quantity of dye in relation to an arbitrarily chosen standard (of a dyeing or print), synonymous with depth 

The exact hue of a single compound dye, and therefore ultimately any mixture with another compound, depends on its absorption characteristics, i.e. both the wavelength and the molar absorption coefficient or height at the absorption maximum, but also of importance is the shape and size (the band width) of the area under the curve. The observed hues given in Table 2.2 are due to the wavelengths of light not absorbed. 

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Figure 2.1 (a) Additive colour mixing (b) subtractive colour mixing... 

Subtractive colour mixing is used for dyes and pigments where the primary colours are red, yellow and blue. 

Procedure. Steam is passed through the steam distillation apparatus for 20-30 min. Check the performance by pipetting 5 ml ammonium-N standard solution into the distillation unit, add 1 drop octan-2-ol, 6 ml magnesium hydroxide suspension and steam distil the released ammonia into 5 ml boric acid solution in a 100-ml conical flask. After approximately 40 ml distillate has been collected over a 5-min period, wash the tip of the condenser into the distillate, add 2-3 drops mixed indicator solution and titrate with 0.005 M H SO until the colour changes from green to purple. A blank distillation/titra-tion is carried out using 5 ml ammonia-free water and subtracted from the standard titre to give a result which should be 5.00 ml. 

Complementary colors n. Two colors which, when mixed together in the proper proportions, resnlt in a neutral color. Colored lights, which are complementary, when mixed additively, form white light and follow the lam of additive color mixture. Colorants, which are complementary, when mixed together, form black or gray and follow the laws of subtractive colorant mixture. With the exception of spectrophoto-metric complementaries, which exist only in theory, colors, which are complementary depend on the illuminant chromati-city considered as the neutral point. McDonald R (1997) Colour physics for industry, 2nd edn. Society of Dyers and Colourists, West Yorkshire, England. 

From experience and by application of the principles of subtractive mixing, the pigments likely to give the colour are chosen. The procedure is described under Pigment selection (above). 

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