Color Mixing
There are two ways to control how much
red, green, and blue light reaches the eye:
“Additive Mixing” Starting with black,
the right amount of red, green, and blue
light are ‘added’ to an image.
“Subtractive Mixing” Starting with white,
the right amount of red, green, and blue
light are ‘subtracted’ from an image.
Additive Color Mixing
By exciting the red, green, and blue
sensitive cones, any color can be
produced by adding together the
three additive primaries (R,G,B).
Mixing the three color sources is known as
“additive mixing” to distinguish it from
mixing paints or dyes (“subtractive mixing”).
Additive Color Mixing
For example, when blue
and green lights overlap,
the blue and green cones
are illuminated, and we
perceive cyan
Additive Color Mixing
green + blue = cyan
red + blue = magenta
red + green = yellow
red + green + blue = white
Additive Color Mixing
red + green/2 = orange
red + green + blue = gray
red + green = yellow
red + green + blue = gray
red/2 + green = lime
red + green + blue = white
Additive Color Reproduction
Color video projectors use additive color mixing
— Projected
red, green, and blue images contribute
RGB components to create color images
R
G
B
Additive Color Mixing Methods
In addition to the superposition method
described above, there are two other
methods of mixing R, G, & B primaries.
- Spatial mixing (as in color TV)
- Temporal mixing (as in digital cinema)
Both rely on limitations of the visual system;
Spatial Mixing (Video Monitor)
Because the visual system has limited spatial resolution,
small areas of different colors are mixed perceptually.
Spatial addressability
of typical monitors goes
from (640 x 480) to
(1600 x 1280) pixels.
y
x
Temporal Mixing (Digital Cinema)
Because the visual system has limited temporal
resolution, rapidly changing colors are mixed
perceptually.
time
time
time
Color Monitors

A number of color monitors exist in most
digital color document systems.
— Different
color monitors are likely to display
the same digital file differently.
Subtractive Color Mixing
Color hardcopy devices can’t use additive mixing because
they aren’t sources of light; they can’t add Red, Green, or
Blue components.
Instead, they use subtractive mixing.
Starting with white light reflected by the
substrate, they subtract the unwanted red,
green, and blue components using cyan,
magenta, and yellow colorants.
Subtractive Color Mixing

The goal is the same; to control the amount of Red, Green,
and Blue light getting to the eyes’ three cone types

Each colorant absorbs 1/3 and transmits 2/3 of white light
white substrate
cyan colorant
“minus red”
magenta colorant
“minus green”
yellow colorant
“minus blue”
Subtractive Color Mixing

Other colors are made by varying the amount of colorant
in each layer.
yellow & magenta = red
yellow
+ magenta/2
orange
yellow
magenta
+ cyan
black
Subtractive Color Reproduction

Color printing uses subtractive color mixing.

Adding black allows more accurate grays, and
conserves the more expensive CMY colorants.
C
Y
M
K
Subtractive Color Imaging

Colors are rendered by different mixtures of
cyan, magenta, and yellow inks printed.
— Gradations
in each channel can be achieved by
halftone marking.
Contone
grayscale
Halftone
grayscale
Subtractive Color Imaging

Process color printing is an example of
subtractive color mixing
— The
spatial addressability of typical printers
goes from 400 spots/in to 3,600 spots/in.
C
Y
M
K
Subtractive Color Imaging

Assumptions:
— White

substrate (or paper) is used
It reflects all red, green, and blue light
— Process

inks are semi-transparent
Each ink absorbs ~1/3 of the visible spectrum
cyan subtracts red, transmits green and blue
magenta subtracts green, transmits red and blue
yellow subtracts blue, transmits red and green