Course on Cartographic Techniques | Cartography Working Group | The Virtual Geography Department

 

Lesson 1: Introduction to Color Theory

I. What is Color?

Color is used to enhance maps in many ways. Color is attractive. Color creates contrast between, and emphasis of mapped features or data. Through contrast, emphasis, and visual appeal, color enhances understanding of the map, thus improves map communication. It is preferrable to use color on maps but it is easy for color to be used incorrectly.

It is important, therefore, for  the cartographer to have a thorough understanding of the many aspects of color in order to use it effectively in map design. This lesson will begin the 4-part series of lectures/exercises on color by looking at the physical properties of color, then exploring how color is seen and perceived by the human visual system as light or as pigment. From there the lesson will cover the primary colors, and illustrate how these can be combined to create other colors.

The reader is also referred to an excellent graphic arts color module entitled Interactive Color, A guide for Color in Computer Graphics. The module was put together by the San Diego Supercomputer Center (SDSC, copyright, 1991) and available by ftp on their public access site for those using Macintosh computers. The module will not work on pc's.

Color is a perceptual phenomenon that results from the way our eyes process Electromagnetic Radiation (see also Color Plate Figure 19.1 in Robinson, et.al, 1995, or the Interactive-Color module, SDSC, 1991, if using a Macintosh). Light is a physical property consisting of waves of energy that are mostly invisible to our eyes. Light energy (Electromagnetic Radiation) propogates through the air as a series of waves of a given "wavelength" where one wave has a single peak and trough. The distance between two successive peaks or troughs is one wavelength.

The Electromagnetic Spectrum extends from the very short wavelengths ( e.g., gamma rays ), measured in millimeters, to the very long wavelengths (e.g., radio and television), measured in kilometers. The units of measure for wavelengths we deal with in sight and remote sensing are: micrometer, nanometer, centimeter, and meter.

There is a small portion of the electromagnetic spectrum, called the Visible Wavelengths. These are the wavelengths that our eyes are sensitive to. This is because the visual receptors of our eyes process or perceive  the sensations of spectral colors or hues, also known as the colors of the rainbow where, for example, white light is split into it's color component parts (the rainbow colors) by a prism.

White light from the entire visible portion of the spectrum, if broken into its component parts, divides into the colors of the rainbow. A prism will separate white light into its component (rainbow) parts. (see also Robinson, et. al., 1995, Color Fig. 19.1) When it rains and the sun is shining, the water acts like a  prism by splitting white sunlight into the shorter wavelengths (relatively): violet, blue, green, followed by the longer wavelengths: yellow, orange, and red. Each time you see a rainbow, note the colors and their order of appearance within the rainbow. Every rainbow will have the same colors, presented in the same order from shorter to longer wavelengths.

As an example, open Adobe Illustrator (note: you can create a rainbow in other graphics software too; the example here and all examples within the color theory module use Adobe Illustrator terminology and menu items). Select Window, Show Gradient, then create an oval or a rectangle using the appropriate toolbar icon and fill with Rainbow. Look at the component parts.

Note: Illustrator adds pink to the end of it's rainbows; a real rainbow ends in red.

1. Color as Light (Spectral Color)

The Electromagnetic spectrum consists of the spectral hues of light. Hue is the specific color, identified by a name, (e.g., red, blue, green, etc.) as they are the component wavelengths of white light. When combined together in equal amounts, they form white light. Color as Light is translucent, that is, we can see through the colors and can project colors over one another to form other colors (see also Additive Primary Colors below, the SDSC Interactive Color module, 1991, and Robinson, et. al, 1995, Color Plate Figure 19.2). This is the mixing of color as light.

Click here for an interactive way to combine the additive primary colors of light. Combine the colors by grabbing each square and overlapping them. Notice  that white is formed by the overlap of all three additive primary colors. (Color mixing java applet created by Phillip Dukes of Bringham Young University, email: g-prd@physics1.byu.edu)

The way we see colors of light is dependent on which colors are being absorbed by white light, for example, we see blue skies when sun angle is high, making the shorter wavelengths dominant while longer wavelengths are absorbed. During the morning and evening when the sun angle is low on the horizon, the shorter wavelengths are absorbed while the longer wavelengths remain  (these wavelengths have a longer distance to travel) and the skies appear orange, red, or pinkish.

Clouds are an equal mix of all spectral hues, thus look white, when there is a 100% mix of all spectral hues, or gray when there is an equal mix of all spectral hues but in less than 100% proportions, that is, when equal amounts of all spectral hues are being absorbed. At night, when there is only the moon and few stars (or stars very far away), the absence of light results in black sky as 100% of all spectral hues are absorbed and 0% remain (a black sky is the absence of all spectral hues).

2. Color as Pigment (Reflected Color)

Pigments are opaque (not transparent) paints or inks placed onto opaque surfaces. These pigments absorb and reflect different amounts of color from white light. Most of the colored objects we see on earth are made up of combinations of reflected wavelengths. Surfaces or objects illuminated by white light absorb differing proportions of visible wavelengths and reflect the remainder. The principle of reflected color is illustrated in the following series of graphics:

a. Sunlight, as described above, is composed of equal amounts of all spectral hues (i.e., the colors of the rainbow). When sunlight, or artificial white light is shone on an opaque surface, certain wavelengths of light (colors) will be reflected off the surface while others will be absorbed by the surface. The "color" we see depends on the type and amount of reflectance of the wavelengths we identify as the spectral hues. White, as shown here, is created by the equal, full-strength (100%) reflectance of all three main spectral hues, that is 100% reflectance of blue, green and red.

b. Medium Grey results from the 50% absorption, 50% reflectance of blue, green and red. Darker grey happens when more than 50% of each primary color is being absorbed and less is reflected. Light grey results when more than 50% of each primary color is being reflected and less is absorbed.

c. Black is the absence of color, or the absence of reflected light, thus the total (100%) absorption of all three primary colors.

d. Other colors can be created by the combined reflectance of certain of the spectral hues. A full strength cyan, for example, is comprised of 100% reflectance of blue and green, and 100% absorption of magenta. A medium strength cyan would result from 50% reflectance of blue and green. More or less than 50% reflectance of blue and green would form darker or lighter cyan, respectively. Note that there must be equal reflectance of blue and green in order for a true cyan to be formed. If more blue were reflected, the resulting color would be blue-green. If more green were reflected, the resulting color would be green-blue. This same concept applies to the formation of all colors as will be demonstrated within Lesson 3: Color Mixing.

e. Magenta (full strength) is formed by the equal reflectance of blue and red. Thus 100% reflectance of blue and red, and 100% absorption of green, forms a full strength magenta.

f. As with cyan and magenta, a full strength yellow is formed by the equal (100%) reflectance of red and green, and 100% absorption of blue.

Spectral Reflectance Curves can be plotted to illustrate the percent of reflectance of certain wavelengths of electromagnetic radiation, where 100% = full reflectance and 0% = full absorption or no reflectance. Notice how the actual color, and the strength of the color is dependent on the combination of varying percentages of reflectance of the primary colors.

Refer also to Robinson, et. al, 1995, Figure 19.2, pg. 343, for more diagrams on reflectance of pigment inks from a paper surface. The figure illustrates the formation of color by a combination of reflectances of different hues in varied proportions.
 

Having seen how color is seen by the eye, and how the mixing of colors of light and pigment results in the creation of other colors, we can further describe the production of color by looking at the primary colors of light and pigment as the formation of color by mixing of light or ink. This can be seen by viewing colors by the relative amount of reflection or absorption of the components of white light or the interaction of white light with opaque, pigment surfaces.
We can discuss two primaries, determined by the way colors are formed: Additive and Subtractive.

A. Additive Primary Colors

The Additive Primary Colors are simplified from the visible spectrum to include only Blue (400-500nm), Green (500-600 nm), and Red (600-700 nm) as shown   here (click on small image) and in Robinson, et., al, Color Figure 19.2 .The mixing of light using additive color primaries is how color is formed on computer monitors and television screens (look closely along the edge of a TV and you may see lines of blue, green and red light).

Note the mixtures arising from the mixing of Additive Primary Colors: (create these by overlapping the additive primary color squares; Java Applet created by Phillip Dukes of Bringham Young University, email: g-prd@physics1.byu.edu ):

These mixtures were demonstrated above in the illustrations on reflected color, or color as pigment. These mixtures result in what are called the subtractive primary colors.

B. Subtractive Primary Colors

The Subtractive Primary Colors, created by the mixing of the Additive Primary Colors, are Cyan, Magenta, Yellow, and also Black,  as shown here (click on small image).  Note that the subtractive primaries are those formed by the mixture of additive primaries as shown in Robinson, et., al, 1995, Color Plate Figure 20.1. This is how paints and inks are mixed and used in painting and printing onto opaque surfaces. Note that the Subtractive Primary Colors mix to recreate the additive primaries: (create the additive primaries by overlapping the subtractive primary color squares. You may need to select "Light Subtraction" to combine the subtractive primary colors; "Light Addition" will combine the additive primaries of light. (Java Applet created by Phillip Dukes of Bringham Young University, email: g-prd@physics1.byu.edu )

These are how printing inks are mixed to create a multitude of  other colors (see also Lesson 3: Color Mixing)

Why are these colors called subtractive primaries? This is because the mixing process involves the reflection of certain primaries and the absorption (or subtraction) of others as was demonstrated earlier. In reality, certain primaries are being subtracted out of white light while others remain to be reflected and seen by the eye. Examples for the Subtractive primaries are shown below:

Red (Magenta + Yellow) = White minus Green (Yellow + Cyan), minus Blue (Magenta + Cyan); in other words, Cyan is subtracted from both Green and Blue, leaving Yellow and Magenta which, when mixed, forms Red. Another way to look at it is that Green and Blue are subtracted from White, leaving only Red.

Blue (M + C) = White - Green (Y + C) - Red (M + Y);
the subtraction of Yellow from both Green and Red leaves M + C  which mixes to form Blue.

Green (Y + C) = White - Red (M + Y) - Blue (M + C)
subtract magenta from both, leaves Y + C which together forms Green.

Black = White - Red - Blue - Green (absence of color)

Or, in condensed form:


Exercise 1

References

Go to Introduction to Color Theory above

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Created 2/20/98 by Laurie A. B. Garo. Last updated 5/21/99 by lg.
The URL for this page is http://www.uncc.edu/lagaro/cwg/color/intro_color.html