IDEAL SUBTRACTIVE COMPLEMENTARIES

Since in the digital realm pairs our ideal subtractive primaries mix to make intermediate colours at full intensity, their mixing relationships can be best summed up on a triangular diagram, as opposed to a circle (Figure 5.6). It would be incorrect however to infer that colours on directly opposite sides of this diagram are exact subtractive complementaries.

Figure 5.6. Mixing relationships of ideal subtractive primaries.

Unlike additive complementaries, which mix to make white, the term complementary in subtractive mixing refers to the colours that mix to make black or grey. It appears to be generally assumed that the subtractive complementary of a colour is identical to its additive complementary. The actual situation is rather complex, even in the ideal subtractive mixing of the digital realm. Consider the two colours below (Figure 5.7), a purple (R 153 G 051 B 255; H= 270) and a green (R 102 G 204 B 000; H= 90). These are additive complementaries (they add up to white light), and they occupy opposite positions on the digital hue circle. They are not however subtractive complementaries - they have some red and green in common. The colour needed to reduce all three components of the purple colour to the same level (making grey) is a different green - R 085 G 255 B 051 (H = 110).

Figure 5.7. Additive and subtractive complementaries of a single colour differing in hue angle by 20°. The additive complementary adds to make white light; the subtractive complementary multiplies the R,G and B brightnesses to make grey.

For the additive primaries "Monitor Red" (R 255), "Monitor Green" (G 255) and Monitor Blue (B 255), all colours lacking that primary are its subtractive complements. On the digital hue circle this includes all colours between 120^o and 240^o from the primary colour. Conversely, for all other pure colour (which contain just two RGB components), the subtractive complementary is the additive primary absent from that colour. The subtractive complementary is the same as the additive complementary for desaturated R,G and B, but is roughly similar but different for other desaturated colours.

We are now ready to look at the role that subtractive mixing plays in the mixing of artists' paints.

Figure 5.8. Colours of twenty two common artists pigments at various thicknesses over a white ground. Photographed colours displayed in YCbCr space using ColorSpace by Philippe Colantoni.

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