Primary Colors
The human eye is sensitive to a narrow band of electromagnetic radiation that lies in the wavelength
range between 400 and 700 nanometers, commonly known as the visible light spectrum. This small
span of electromagnetic radiation is the sole source of color. All of the wavelengths present in visible
light form colorless white light when they are combined, but can be refracted and dispersed into their
individual colors by means of a prism.
The colors red, green, and blue are classically considered the primary colors because they are
fundamental to human vision. All other colors of the visible light spectrum can be produced by
properly adding different combinations of these three colors. Moreover, adding equal amounts of
red, green, and blue light produces white light and, therefore, these colors are also often described
as the primary additive colors.
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Primary Additive Colors
Explore how the primary additive colors interact with one
another to form new colors.
As illustrated by means of the overlapping color circles in Figure 1, if equal portions of green and
blue light are added together, the resultant color is cyan. Similarly, equal portions of green and red
light produce the color yellow, and equal portions of red and blue light yield the color magenta. The
colors cyan, magenta, and yellow are commonly termed the complementary colors because each
complements one of the primary colors, meaning that the two colors can combine to create white
light. For instance, yellow (red plus green) is the complement of blue because when the two colors
are added together white light is produced. In the same way cyan (green plus blue) is the
complement of red, and magenta (red plus blue) is the complement of green light.
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Primary Subtractive Colors
Examine how the primary subtractive colors combine to
form the primary additives, as well as black, the absence of
all color.
The complementary colors (cyan, yellow, and magenta) are sometimes alternatively referred to as
the subtractive primaries. This is because each one can be formed by subtracting one of the
primary additives (red, green, and blue) from white light. For example, yellow light is seen when all
blue light is removed from white light, magenta when green is removed, and cyan when red is
removed. Consequently, when all three of the subtractive primary colors are combined, all of the
additive primary colors are subtracted from white light, which results in black, the absence of all
color.
Thus far this discussion has centered on the properties of visible light with respect to the addition
and subtraction of transmitted visible light, which is often visualized on the screen of a computer or
television. Most of what is actually seen in the real world, however, is light that is reflected from
surrounding objects, such as people, buildings, automobiles, and landscapes. These objects do not
produce light themselves, but emit color through a process known as color subtraction in which
certain wavelengths of light are subtracted, or absorbed, and others are reflected. For instance, a
cherry appears red in natural sunlight because it is reflecting red wavelengths and absorbing all
other colors. The series of photographs presented below in Figure 2 helps further illustrate this
concept.
In the first photograph on the left, a playing card, a green bell pepper, and a cluster of purple grapes
are illuminated with white light and appear as one would expect to see them under natural lighting. In
the second photograph, however, the objects are illuminated with red light. Note that the playing
card reflects all of the light that strikes it, while only the grape stem and highlights on the grapes and
pepper reflect the red light. The majority of the red light is being absorbed by the grapes and pepper.
The third photograph shows the objects under green illumination. The different radiation wavelength
causes the symbols on the playing card to appear black and the body of the card to reflect green
light. The grapes reflect some green light, while the pepper appears normal, but with green
highlights. The fourth photograph illustrates the objects under blue illumination. In this situation, the
grape cluster appears normal with blue highlights, but the stem is invisible because it blends into the
black background. The body of the playing card reflects blue light and the symbols appear black,
while the pepper only reflects blue light as highlights.
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Color Filters
Investigate how color filters operate to alter the apparent
color of objects visualized under white light and
monochromatic illumination.
The human eye can perceive very slight differences in color and is believed to be capable of
distinguishing between 8 to 12 million individual shades. Yet, most colors contain some proportion of
all wavelengths in the visible spectrum. What really varies from color to color is the distribution of
those wavelengths. The predominant wavelengths of a color determine its basic hue, which can be,
for example, purple or orange. It is the ratio of the dominant wavelengths to other wavelengths,
however, that determines the color saturation of the sample and whether it appears pale or deeply
saturated. The intensity of the color and reflectivity of the object being imaged, on the other hand,
determine the brightness of the color, which controls, for instance, whether something appears dark
or light blue.
Over the years, various classification systems have been devised to systematically express color in
terms of these concepts. One of the most widely accepted has been the Munsell Color Tree, which
appears below in Figure 3. As illustrated, each color in this system is represented by a distinct
position on the tree. Hue color value is represented by placement on the circumference, saturation
by the horizontal distance of the color from the central axis, and brightness by the vertical position on
the trunk.
When learning about color, it is also important to consider pigments and dyes, which are responsible
for much of the color that appears on Earth. For instance, the natural protein pigments that are
contained in eyes, skin, and hair reflect and absorb light in such a way that creates a beautiful
diversity of appearances in the human race. In order to achieve a similar diversity of color in
inanimate objects, such as automobiles, airplanes, and houses, they are frequently coated with
pigment-containing paints and portray different shades through the process of color subtraction.
Printed items, such as books, magazines, signs and billboards, create colors in the same
fundamental way, but through the help of dyes or inks, rather than pigments.
All color photographs, and other images that are printed or painted, are produced using just four
colored inks or dyes--magenta, cyan, yellow (the subtractive primaries) and black. Mixing inks or
dyes of these colors in differing proportions can produce the colors necessary to reproduce almost
any image or color. The three subtractive primaries could, in theory, be used alone. However, the
limitations of most dyes and inks make it necessary to add black to achieve true color tones.
When an image is being prepared for printing in a book or magazine, it is first separated into the
component subtractive primaries, either photographically or with a computer as illustrated above in
Figure 4. Each separated component is then made into a film that is used to prepare a printing plate
for that color. The final image is created by sequentially printing each color plate, one on top of
another, using the appropriate ink to form a composite that recreates the appearance of the original.
Paint is produced in a somewhat similar manner. Again, only the subtractive primaries and black are
required. Base pigments containing these colors are mixed together to form the various colors used
in final paint preparations.
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Color Separation
Discover how the subtractive primaries are separated from an image
and are made into color plates that can be used to produce
realistically colored prints.
A clear understanding of the color concepts previously discussed is extremely important when using
a microscope to view and capture color images. Microscope light sources are usually tungsten-
halogen bulbs that can emit a bright light with a color temperature around 3200 Kelvin. To the
observer, this appears as white light that can be absorbed, refracted, reflected, polarized, and/or
transmitted by a specimen on the microscope stage. The rules of primary colors apply to how the
specimen interacts with microscope light and what colors are displayed as the sample is visualized
in the eyepieces. The same rules also apply to the film used to capture photomicrographs.
