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"Why is my blue purple?"
- A Few Notes on Color
The short answer:
RGB Blue = 0 Red, 0 Green, 255 Blue (RGB) = 88% Cyan, 77% Magenta
0% Yellow, 0% Black (CMYK). When you mix roughly equal parts red
(magenta) and blue (cyan), that's purple, folks. If you want blue,
pick something with less magenta in it.
The Long answer:
When an RGB image file is sent to a four-color printer (just
about every printing device out there from a lowly ink-jet to
a megabuck color copier or printing press works this way), the
color values are converted from RGB to CMYK. If you're ever curious
what the conversion values are, just pull up Photoshop's color
picker and you'll find both values listed for your color.
So why can't you just print in RGB? The answer is printers
create images with pigment (toner, ink, etc.) as opposed to beams
of light on your monitor and light and pigment don't mix the same
way.
Your computer monitor creates an image by projecting beams of
light onto the screen made up of three primary colors: reg, green,
and blue. If you add equal amounts of them together at full intensity,
you get white light. This is referred to as an additive color
model.
Printers, on the other hand, use a four-color process. The primary
colors are cyan, magenta, and yellow, with black added to darken
the other three primaries. When a printer mixes together equal
parts of cyan, magenta, and yellow at full intensity, you get
a dark color that could pass for black. This is referred to as
a subtractive color model.
The color range, or gamut, of each color model produces a finite
number of colors. The added kicker is the possible color range
of CMYK is smaller than that of RGB. Net result: there are RGB
colors that just do not exist in CMYK. What happens when an out
of range RGB color is converted to CMYK? The computer is forced
to make the best possible match. Sometimes it'll come close, other
times it'll be way off.
What about color calibration?
Color calibration was created to try and take some of the guesswork
out of getting what you see on screen to match what you get out
of the printer. The system generally works by comparing profiles
of each devices known color characteristics to get some degree
of consistency in a world with so many variables (CMYK vs. RGB,
lighting conditions, monitor settings, printing technologies,
etc.). No system is ever going to be 100% accurate, but when combined
with good color management, you can produce much more accurate
results than if you just picked the default blue in PowerPoint
and clicked print.
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