U n d e r s t a n d i n g
C o l or

Additive vs Subtractive Color

All forms of image creation is based on the ability to create a large number of colors from a small subset. For example, the computer monitor uses just 3 colors. In painting, the artist uses a small palette of colors which are combined to obtain a wide range of colors. How colors combine, however, differs depending upon the medium.

The two basic forms of mixing are additive and subtractive. Light (e.g. from a monitor) is additive while paints tend to be subtractive.

With an additive medium such as light, given any two colored light sources A and B, when mixed simply produce the result of the combination A + B. With a subtractive medium such as paints, colors A and B are mixed to obtain only what is common to A and B. One can think about adding what is taken out. For example, if Ac is the optical complement of A then that means that a material with color A must absorb color Ac. Thus, when A and B are mixed, we have that Ac+Bc is absorbed. The color that appears is the complement of (Ac+Bc).

Subtractive media behave like filters in sequence whereas additive media behave like filters in parallel.

A result of this property is that metamers do not always mix with other colors to produce the same color. Consider 3 different light spectra: S1, S2, and S3. Suppose that S1 has color A and S2 and S3 are metamers of color B. Since light is additive, when we mix S1 and S2 we will obtain the same color as when we mix S1 and S3. Additive colors are easy to work with because it is safe to ignore the underlying differences in spectra.

However, this is not true for subtractive colors. Mixing S1 and S2 may not produce the same color as S1 and S3:

Pointillism

Paints can be made to behave as additive colors. Rather than mixing the colors, artists have been known to create colors by using many individual dots of pure color. From a distance, your eye combines the color to see the additive result. This technique is referred to as pointillism.

Paints reflect and absorb light at different wavelengths because of the presence of many tiny colored particles called pigments. Unlike filters, paints are really not completely subtractive. The amount of light that is reflected by a combination of paints is actually intermediate between the two components.

Paints depend upon the light that is reflected off of them. As a result, the same paint will look different under different lighting conditions. For example, a particular color of paint will look different in the sunshine than under artificial light. Different colors of paints also may look the same (i.e be metamers) under one lighting condition but not another. Thus we can only talk about metamers with respect to specific lighting conditions.

See Examples of pointillism in the arts and George Seurat.

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