GEOG 486
Cartography and Visualization

Part IX: Color Spaces and Specification

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Color specifications are ways of describing color. Unfortunately, the everyday language we use to talk about color is rather imprecise. For example, if we ask you to think about the color purple, some of you might imagine the color of red grapes, others might imagine the color of a Minnesota Vikings jersey, and still others might think of the purple of lilac flowers. For this reason, scientists and other people who regularly work with color have developed a number of mathematical systems for describing color. These mathematical systems are more formally known as color spaces. In this section, we will focus on two color spaces that are directly related to map production technologies: CMYK and RGB. We will talk about other types of color spaces and specifications in the Color Spaces concept gallery item in Lesson 2.

We are able to sense color because our eyes contain groups of cells (called cones) that are sensitive to light at specific wavelengths. People with color vision impairment (see more about this in the Lesson 2 concept gallery section on Color Vision Impairment) are missing some of these specialized cells. This light that our eyes and brain transform into what we call color can come from two sources: light that is emitted from a light source (e.g. the sun, a computer projector or a computer monitor) or light that is reflected from a surface (e.g. paper or some other media that is covered with pigments that absorb and reflect certain wavelengths of light).

The CMYK (cyan, magenta, yellow and black) color specification system is used in production technologies that use reflections from pigments to create color. CMYK colors are also known as process color. Because the colors we see when we look at a map created with ink (i.e., pigments) are determined by the wavelength of the light that is left after the pigment has absorbed other wavelengths, we call pigment-based media subtractive media (i.e., we perceive the color red when the ink on the page absorbs, or subtracts, green and blue wavelengths) (see Figure 1.9.1). Although in theory, if we mix cyan, magenta and yellow together, we should get black (i.e., all wavelengths of light are absorbed), it is very hard to obtain pure pigments, so some amount of light gets reflected. For this reason, we also specify black in this color system, particularly for those cases where designers would like to have a very pure black. This system uses percents of ink to specify colors. For example, a bright red color might be specified as: C: 0%, M: 90%; Y: 60%, K: 0%.

A graphic to show and describe additive and subtractive color spaces
Figure 1.9.1 Additive and subtractive primaries for reflective and transmissive media

The RGB color specification system allows designers to specify how much light of a particular wavelength (i.e., the intensity) should be emitted. This color system is called additive color because it is based on the stimulation of additional types of cells (the different types) of cones with different wavelengths of light. RGB colors can be specified with a range of 0 to 255, with 255 being the highest intensity of light and zero the lowest intensity of light (e.g., a vibrant red in RGB would be specified as R: 255, G: 0, B: 0). This 256-level system is an artifact of computer technologies (i.e., there are 8 bits in 1 byte, and those 8 bits can be used to specify 256 levels).

A final color specification that is related to map production technology is the specification for spot colors. Spot colors, as you may recall from Part V: Print and Electronic Media, are premixed pigments used in offset printing. This specification is similar to the one used for CMYK in that you specify a percent of ink that you want to use for a particular symbol that uses a spot color.

Now you've learned a bit about the two main methods cartographers use for specifying colors. We will go into more detail on the RGB and CMYK color models in the Color Spaces concept gallery item in Lesson 2, and we will look at why it is not possible to create all colors in all color specification systems.


Recommended Readings

If you are interested in investigating this subject further, I recommend the following:

  • Brown, A. and W. Feringa. (2003) Colour Basics for GIS Users. Harlowe, England: Pearson Education Limited.