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Subtractive color is a general term that covers the mixing of paints, dyes, inks

Subtractive color is a general term that covers the mixing of paints, dyes, inks, and natural colorants to create colors which absorb some wavelengths of light and reflect others. The color that an opaque object appears to have is based on what parts of the electromagnetic spectrum are reflected by it, or conversely by what parts of the spectrum are not absorbed.

Anything that is not additive color is subtractive color.

Color is not an absolute, but depends on the details of human color vision, which varies between individuals. Although color can be measured by instruments, such instruments are simply emulating a particular individual's vision.

Consider a "red" apple. When viewed under a white light, it appears red. But this doesn't mean that it emits red light, as would be the case in additive color. If it did, you would be able to see it in the dark. Rather, it absorbs some of the wavelengths that make up white light, reflecting only the wavelengths that humans see as red. Humans perceive the apple as red because of how their eye works, and how their brain interprets information from the eye.

It takes three things to see color: a light source, a sample, and a detector (which can be an eye).

Color Printing
In color printing, the primary inks used are cyan, magenta, and yellow. Cyan is the opposite of red, meaning that cyan acts like a filter that absorbs red. The amount of cyan applied to a paper will control how much red will show. Magenta is the opposite of green, and yellow the opposite of blue. With this knowledge an infinite number of color combinations are possible. This is how artwork reproductions are mass produced, though for various reasons a black ink is usually used as well (see limitations). This mixture of cyan, magenta, yellow and black is commonly called CMYK. CMYK is therefore an example of a subtractive color space, or rather a whole range of color spaces, since inks can vary, and the effect of the inks depends on the paper used. The main reason that black ink (as in CMYK) is used in addition to cyan, magenta, and yellow, is that the latter inks usually cannot be combined to create a true black. No colored inks will absorb all wavelengths which might appear reddish, for example, meaning that all practical CMY mixtures, mixed at full intensity, will produce an off-black result. The colored inks are therefore printed first to produce the hue, while the black ink is used to produce the value.

Opaque Paints
Mixing of opaque paints such as oils is covered by the Kubelka Munk theory which gives a linear combination of the K/S values for each frequency of the component colors, where K/S is related to the reflectance by


 * $${K \over S} = {\left( 1 - R \right) ^2 \over 2R}$$

To cover a gamut equivalent to RGB or CMYK eight primaries are required: red, green, blue, cyan, magenta, yellow, white and black.

Practical use of Subtractive Color in Color printing/Graphic Arts/Art Reproduction
The untrained eye cannot distinguish a reproduction from the original artist's painting from a distance, but the reproduction is just a simulation of the original. For example, as seen above, CMYK reproduction does not contain the actual green tint pigment in a reproduction of Michelangelo's painting. The actual painting may well contain a true green pigment which reflects the green wavelength from the visible spectrum, and absorbs all others. Similarly, it is possible to have an orange pigment that will reflect from the visible spectrum the exact orange wavelength that we can normally see in the rainbow.

Additionally, not all colors can be simulated in print with the CMYK color space. For example, the system is unable to reproduce the colors gold and silver. If these colors are necessary, they are added as the fifth, sixth or other color separation. Sometimes six "normal" subtractive mixing colors (Hexachrome system) are used in modern printing to extend the space of truly printable colors and hence achieve more realistic and professional results. To faithfully reproduce the color of a flag or a logo and sometimes for economic reasons, it is possibile to include special separation layers of exact colors from a palette, as in the Pantone system - it can be also considered as using subtractive color, but such color is usually used for monochromatic areas and not mixed in print.