PIGMENT MIXING PATHS           

The following principles may be helpful in suggesting the likely path of pigment mixtures. All nine principles could be examined in greater depth, but hopefully this short summary will guide students to making their own more detailed studies.

(1) Pairs of pigments close in hue to the ideal subtractive primaries tend to mix on lines that bow outwards on the colour wheel, passing through mixtures that are relatively high in chroma.

Figure 6.7. Mixtures of (A) Lemon Yellow (B) Quinacridone Magenta and (C) Pthalocyanine Blue (Green Shade). Photographed colours in YCbCr space, viewed from below. Each pair of primary colours mixes along a line that is convex outwards, keeping relatively close to full chroma.

(2) Pairs of pigments that are far from the hue of the ideal subtractive primaries mix along lines that are straight or bow inwards on the colour wheel, passing through mixtures that are relatively low in chroma.

Figure 6.8. Mixtures of (A) Cadmium Orange, (B) Ultramarine Blue and (C) Pthalocyanine Green (Yellow Shade). Photographed colours in YCbCr space, viewed from below. Colours on each mixture line move away from full chroma.

3) Among pairs of pigments in the same general sector of the hue circle, pairs that are close in hue tend to mix along lines of higher chroma than pairs that are more distant.

Subject to the overall pattern described under the first two principles, and also to the chroma of the pigments concerned, pairs of colours close in hue tend to mix along lines of higher chroma than pairs that are more remote in hue. Figure 6.9 shows mixtures of one pigment close to ideal primary yellow (Cadmium Yellow Light) with two different pigments remote from the ideal subtractive primaries (Cadmium Red Deep and Cadmium Red). Both pigment pairs mix through a range of intermediate orange hues along lines that happen to be, in this representation, nearly straight in plan view. The pair that are closer in hue, Cadmium Yellow Light and Cadmium Red, mix along a line of higher chroma than the pair that are more remote in hue. In this particular case the difference is exacerbated by the lower chroma of Cadmium Red Deep.

Figure 6.9. Effect of hue difference on mixing paths. Left. Mixtures of Cadmium Yellow Light (A) with Cadmium Red Deep (B) and Cadmium Red (C). Right. Plan and side views of photographed colours in YCbCr space. (Note that in all of these diagrams the photographed colours depend on exposure, which needs to be kept low for images depicting Cadmium Yellow to avoid "clipping" against the roof of the RGB gamut. This is why the cadmium reds are duller here than in some of the other figures).

The next three principles relate to the lightening and darkening of colours by the addition of white and black. Note that in addition to these effects on chroma, adding black and white also generally causes a hue shift (Principle 8).

(4) Adding white progressively to a dark transparent pigment tends to increase the chroma of the mixture up to a certain point, beyond which the chroma diminishes. Thinning a layer of a dark transparent pigment over white follows a similar path, but the maximum chroma attained is greater than that attained by adding white.

Figure 6.10. Effect of lightening Permanent Alizarin Crimson by adding white paint (A) and by glazing over white (B). The second method always attains a higher maximum chroma. Both processes also involve a counterclockwise shift in hue towards magenta (right).

(5) Adding white progressively to an opaque pigment tends to steadily diminish the chroma.

Figure 6.11. Effect of adding white to opaque pigments. Unlike transparent pigments, addition of white to an opaque pigment like Cadmium Red Deep does not increase the chroma, but steadily reduces it. This mixture also shows a slight drift towards crimson on the CbCr plane (C).

(6) Mixtures of two colours of the same lightness do not necessarily lie along a line of uniform lightness.

Two plausible assumptions that students often make are that when opaque paints mix, the most neutral mixture will be the darkest, and that one can reduce the chroma of a paint without changing its lightness by adding a grey of the same lightness. Both assumptions are incorrect (Figure 6.12).

Figure 6. 12  Mixture of opaque complementary colours. Cadmium Red and Cobalt Green (A) are additive complementaries and near pigmentary complementaries (B). Note that the most neutral mixture is not the darkest, and that this near grey neutral mixes with red of similar lightness along a curved line, not a line of uniform lightness (white line).

(7) Adding black to mixed colours generally reduces the chroma more rapidly than the lightness, drawing the mixture along a curved path rather than a straight line to black.

Figure 6.13. Effect of adding black to a mixture of W&N Permanent Alizarin Crimson and Flake White.

(8). Adding white, thinning over white, and adding black all usually cause a hue shift, the magnitude and direction of which depend on the pigments involved.

For example, all yellow pigments shift towards green when white and especially
when black are added, although the magnitude of the effect varies between different
yellows.

Figure 6.14. Effect on hue of adding black to Cadmium Yellow Light (A), and white to Permanent Alizarin Crimson (B). Red arrows show the direction of hue shift.

Finally, a principle relating to the general difference between the behaviour of transparent and opaque pigments.

(9). Opaque colours may lighten or darken a mixture, but transparent colours always darken.

Transparent colours always darken mixtures, because they work by removing light. Opaque colours tend to lighten a darker mixture, but their effect on lightness is more conditional, depending as it does on the balance of their effect of absorbing some wavelengths with their effect as added reflectors of the remaining wavelengths (Figure 6.15).

Figure 6.15. Effect of adding Cadmium Red (B) and Permanent Alizarin Crimson (C) to a dark grey mixed from Flake White and Charcoal Grey.

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Next: Part 7: Hue