A color model is an orderly system for creating
a whole range of colors from a small set of primary colors. There
are two types of color models, those that are subtractive
and those that are additive. Additive color models use light
to display color while subtractive models use printing inks.
Colors perceived in additive models are the result of transmitted
light. Colors perceived in subtractive models are the result of
reflected light.
The Two Most Common
Color Models
There are several established color models used
in computer graphics, but the two most common are the RGB model
(Red-Green-Blue) for computer display and the CMYK model (Cyan-Magenta-Yellow-blacK)
for printing.
RGB Color Model
Additive color model
For computer displays
Uses light to display color
Colors result from transmitted light
Red+Green+Blue=White
Subtractive color model
For printed material
Uses ink to display color
Colors result from reflected light
Cyan+Magenta+Yellow=Black
Notice the centers of the two color charts. In
the RGB model, the convergence of the three primary additive colors
produces white. In the CMYK model, the convergence of the three
primary subtractive colors produces black.
In the RGB model notice that the overlapping of additive colors
(red, green and blue) results in subtractive colors (cyan, magenta
and yellow). In the CMYK model notice that the overlapping of subtractive
colors (cyan, magenta and yellow) results in additive colors (red,
green and blue).
Also notice that the colors in the RGB model are much brighter than
the colors in the CMYK model. It is possible to attain a much larger
percentage of the visible spectrum with the RGB model. That is because
the RGB model uses transmitted light while the CMYK model uses reflected
light. The muted appearance of the CMYK model demonstrates the limitation
of printing inks and the nature of reflected light. The colors in
this chart appear muted because they are displayed within their
printable gamut (see below).
Additive vs. Subtractive Color Models
Since additive color models display color as a
result of light being transmitted (added) the total absence of light
would be perceived as black. Subtractive color models display
color as a result of light being absorbed (subtracted) by the printing
inks. As more ink is added, less and less light is reflected. Where
there is a total absence of ink the resulting light being reflected
(from a white surface) would be perceived as white.
Each color model has is own gamut (range) of colors
that can be displayed or printed. Each color model is limited to
only a portion of the visible spectrum. Since a color model has
a particular range of available color or gamut, it is referred to
as using a "color space". An image or vector graphic is
said to use either the RGB color space or the CMYK color space (or
the color space of another color model). Some graphic applications
present the user with more than one color model for image editing
or illustration and it is important to choose the right one for
the task. The whole point of this article is to explain the difference
between the two color models so you choose the right one for the
job. For you work to display at its best, choosing the right color
model is critical.
RGB Color
The RGB model forms its gamut from the primary
additive colors of red, green and blue. When red, green and blue
light is combined it forms white. Computers generally display RGB
using 24-bit color. In the 24-bit RGB color model there are 256
variations for each of the additive colors of red, green and blue.
Therefore there are 16,777,216 possible colors (256 reds x 256 greens
x 256 blues) in the 24-bit RGB color model.
In the RGB color model, colors are represented by varying intensities
of red, green and blue light. The intensity of each of the red,
green and blue components are represented on a scale from 0 to 255
with 0 being the least intensity (no light emitted) to 255 (maximum
intensity). For example in the above RGB chart the magenta color
would be R=255 G=0 B=255. Black would be R=0 G=0 B=0 (a total absence
of light).
CMYK or "Process
Color"
The CMYK printing method is also known as "four-color
process" or simply "process" color. All of the colors
in the printable portion of the color spectrum can be achieved by
overlapping "tints" of cyan, magenta, yellow and black
inks. A tint is a screen of tiny dots appearing as a percentage
of a solid color. When various tints of the four colors are printed
in overlapping patterns it gives the illusion of continuous tones
- like a photograph:
The
CMYK model forms its gamut from the primary subtractive colors
of cyan, magenta and yellow. When cyan, magenta and yellow inks
are combined it forms black - in theory. However, because of
the impurities in ink, when cyan, magenta and yellow inks are
combined it produces a muddy brown color. Black ink is added
to this system to compensate for these impurities.
In the CMYK color model, colors are represented
as percentages of cyan, magenta, yellow and black. For example in
the above CMYK chart the red color is composed of 14% cyan, 100%
magenta, 99% yellow and 3% black. White would be 0% cyan, 0% magenta,
0% yellow and 0% black (a total absence of ink on white paper).
When In Doubt, Save Images As RGB
The RGB model displays a much larger percentage
of the visible spectrum than the CMYK model and, as a result, has
a wider gamut. Once an image has been converted from RGB to CMYK
and brought into printable gamut, the extra RGB data will be lost.
One can retain out-of-gamut areas in CMYK images
and leave it to the computer to bring the colors into gamut at printing
time. However this requires the computer to make the conversion
from RGB to CMYK and this doesn't always work out as well as one
might expect. Conversion between color models is not always a good
idea.
Because of this fact, you want to scan or shoot
images (with a digital camera) using the appropriate color model
for their primary purpose. If the images will be used primarily
for print then use CMYK. If they will be used primarily for screen
displays, then use RGB. You can always convert from RGB to CMYK
(or vise-versa) but it is best not to.
If you can afford the time, money and disk space
to scan or shoot both versions of an image where both are needed,
then this is the best solution. This is especially true if you will
be using the same images for both printed material (such as a catalog)
and the web (such as an online catalog). A little planning can go
a long way here.
But if you are not sure, then I generally recommend
saving images in RGB mode and creating CMYK copies for printed material
as needed.
Use RGB For Screen Displays and CMYK For Print
It is important to choose the right color model
for the job. If your images will be printed, then convert them to
CMYK and manually bring them into gamut before printing. If your
images are to be displayed on a computer, then make sure you use
RGB color so the full gamut will be available for display. Because
both models can be available at the same time while using an application,
it is easy to make a mistake and choose the wrong palette or set
of color swatches.
