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A Little Bit More About Color
Color Depth
Color "depth" is defined by the number
of bits per pixel that can be displayed on a computer screen. Data
is stored in bits. Each bit represents two colors because it has
a value of 0 or 1. The more bits per pixel, the more colors that
can be displayed. Examples of color depth are shown in the following
table:
Color Depth
1 bit color
4 bit color
8 bit color
24 bit color |
No. of Colors
2
16
256
16,777,216 |
Color Mode
Indexed Color
Indexed Color
Indexed Color
True Color |
Palette
Yes
Yes
Yes
No |
Determining Color Depth
Since each bit represents 2 colors, it is easy
to work out the number of colors for the various color depths. The
number of possible colors would be 2 to the power of the number
of bits per pixel:
A color depth of 4 bits would be 2 times itself 4 times:
2 x 2 x 2 x 2 = 16 colors
A color depth of of 8 bits would be 2 times itself 8 times:
2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 = 256 colors.
A color depth of of 24 bits would be 2 times itself 24 times:
2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x
2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 = 16,777,216 colors
True Color
Images are known as "True Color" where
each pixel is defined in terms of its actual RGB or CMYK values.
Every pixel in a a true color image has 256 possible values for
each of it's red, green or blue components (in the RGB model) or
cyan, magenta, yellow and black (in the CMYK model). Because there
are 256 possible values for each RGB or CMYK component, then RGB
true color would have a 24-bit color depth and CMYK true color would
have a 32-bit color depth. There are millions of possible colors
for each pixel in a true color image. That's why it is called "True
Color".
RGB images are derived from the 3 primary colors
of red, green and blue. In 24-bit RGB color, each red, green and
blue component is 8 bits long and has 256 variations in intensity.
These variations are represented in a scale of values ranging from
0 to 255 with 0 having the least intensity and 255 having the greatest.
When the 3 components are combined there are 256 x 256 x 256 possible
combinations or 16,777,216 possible colors.
For example, white would be composed of maximum intensity of red,
green and blue light (R=255 G=255 B=255) and black would be composed
of zero intensity of red, green and blue light (R=0 G=0 B=0). Cyan
would be composed of maximum intensity of blue and green light and
zero intensity of red light (R=0 B=255 G=255). Magenta would be
composed of maximum intensity of red and blue light and zero intensity
of green light (R=255 G=0 B=255):
True Color and The CMYK Color Model
Images using the CMYK color model are also true
color. CMYK images are derived from the 3 primary colors of cyan,
magenta and yellow plus black. In 32-bit CMYK color, each cyan,
magenta, yellow and black component is also 8 bits long and has
256 variations in intensity. Each pixel in a 32-bit CMYK image is
one of 256 x 256 x 256 possible colors x 256 variations of black.
A mix of 100% each of cyan, magenta and yellow produces black, so
the black component is extra (more
here). Even though there are more bits per pixel in the CMYK
model, in reality it has a smaller color "space" or gamut
than RGB (more here).
Many graphics programs support both color models.
Indexed Color
Images which do not define colors in terms of their
actual RGB or CMYK values and which derive its colors from a "palette"
are known as "Indexed Color". The color palette of an
indexed color image has a fixed number of colors. Because the palette
is limited to a maximum of 256 colors, it is not possible for an
image to look as realistic as it can using RGB or CMYK. Hence, they
are not true color. This type of color is known as "Indexed
Color" because colors in the palette are referenced by index
numbers which are used by the computer to identify each color. Some
file formats restrict the number of colors to fewer than 256 . The
GIF format is one
such format and has a color depth of 8 bits per pixel or less. GIF
files use indexed color and allow a maximum of 256 colors. TIFF
files can be stored as indexed color or true color. A sample palette
is shown below:
Windows 16 Color Palette
with Index Numbers
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Grayscale Images
Grayscale images have a maximum color depth of
8 bits. The reason for this can also be worked out easily. When
defining shades of gray in terms of RGB, each of the 3 red, green
and blue components must be equal to each other. Examples
of grays are R=192 G=192 B=192, and R=128 G=128 B=128. Since all
three components must be equal for any shade of gray there are only
256 possible combinations. Thus, grayscale images have a maximum
color depth of 8 bits. A complete 256 color grayscale palette is
shown in the sample images below.
It is possible to create a grayscale image with a 4-bit color depth
or less. Some software packages allow the user to reduce the number
of colors in the palette from 256 to 16 or 2, though one would hardly
classify a 2-color image as grayscale.
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Note:
The palette images below have actual color values in them. If
you wish you can download them and save them to your hard disk.
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| To download in Windows®: |
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Right click image and select "Save Image
As..." |
| To download in Mac®: |
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Drag image onto the Desktop |
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Sample Images
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Palettes
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1 bit color:
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2 color palette:
black and white |
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1 bit color:
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2 color palette:
blue and white |
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4 bit color:
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16 color palette:
Windows 16 color palette
used for screen graphics |
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8 bit color:
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256 color palette:
  |
 
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8 bit grayscale:
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256 shade grayscale palette:
  |
 
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24 bit color:
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(No palette used) |
Spot Color vs. Process
Color
So far, all that has been described in this article
applies to computer displays. But what about printing? What if you
want to print a job without using process
color? What if you want to print a two or three-color job? What
if you want to use a special color ink such as on a company logo?
In this case use "spot color". Spot color is the use of
pre-mixed colors rather than by combining color from cyan, magenta,
yellow and black.
To ensure consistency of color the printing industry has adopted
standards for spot color. There are several spot color systems but
the most widely used is the PANTONE® system. Pre-mixed colors
are selected from swatch books. These same color swatches appear
in graphics programs.
When printing a spot color job, it is best to use
PANTONE colors to ensure consistency of color. However, you may
want to mix your own color for a company logo. In this case have
the printer contact his ink sales representative and have the ink
company formulate the color for you. This service should cost little
or nothing more than the ink itself and can go a long way to establishing
that "special image" for your clients.
Many companies use a combination of four color process and spot
color in the printing of their product catalogs and sales brochures.
The photographs of the products themselves are printed using four
color process and the company logo with a standard PANTONE spot
color or custom color. This would be a 5-color job. There are many
variations on this theme but the basic printing methods are spot
and process.
Related Articles
Images and text Copyright © 1998 by Mike Doughty, All
Rights Reserved. This tutorial reproduced here with permission.
For more tutorials and help, visit Mike's Sketchpad at http://www.sketchpad.net
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