Color Models
green, blue) color model, but graphic arts and design use other color models,
such as HLS (hue, lightness, saturation) or HSV (hue, saturation, value), and
in printing, the CMYK (cyan, magenta, yellow, black) model predominates.
Great books have been written about color; this section presents a quick survey
The RGB Model
In the RGB model, the three colors are additive. The more of each color you
add, the closer the resulting color is to white. This is the way light-produced
colors work; turning on the red, green, and blue phosphors of a television
screen produces white, as does shining lights of red, green, and blue upon a
stage.
color model.
The frontispiece of Volume V of Inside Macintosh shows a color cube that
represents the values possible in an RGB system. The Figure below is a
black-and-white representation of that cube.
The RGB color cube
Starting at one corner, with 0 values for each color, is black. Increasing any
one of the values produces shades of that color, increasing its saturation.
Increasing all three values equally generates a diagonal line across the cube
toward full value (65,535) for each, which is white. Values on that diagonal
are shades of gray; values off the line in any direction are colors. For example,
pink in the RGB model would be full red with some equal amount of green and
blue, in effect moving from the black corner of the color cube up along the edge
to full red, then traversing a diagonal across the top face from red toward
white, as illustrated in the figure below.
Getting to pink
The CMYK Model
In the CMYK model, which is used by printers, the three colors and black are
subtractive: Increasing values moves the result closer to black. This model is
intuitive for printing, which is usually done on white paper-to get white,
don't print anything. In theory, black could be achieved by mixing full values
of cyan, magenta, and yellow, but purity in chemicals is more problematic
than purity in light, and four-color print processes use black as well. The
which are the fractional parts of fixed values, as described in the section
Using Conversion Facilities under the section en titled
Note in the figure below that cyan, magenta, and yellow are complements of
red, green, and blue.
Cyan, magenta, and yellow on the color cube
The HLS and HSV Models
colors, as in the RGB and CMYK models. The HLS and HSV models separate
color, or hue, from brightness and saturation.
Brightness is a measure of the amount of black in a color (the less black, the
brighter the color); saturation is a measure of how much white it contains
(the less white, the more saturated the color). Hue is indicated by an arbitrary
assignment of numbers to colors. The amount of that hue is indicated by a
saturation value, and the brightness of the color is a third value. The best
representation for such a system is an inverted cone, as shown in the figure
below, in which hues vary around the perimeter, where they are most highly
saturated, and brightness increases from the tip of the cone to the disk. The
gray line from black to white begins at the tip and runs up through the cone to
the center of the disk.
The HLS/HSV color cone
shown full face, the hues are at their most saturated around the rim, and the
brightness line down the cone is controlled by the scroll bar at the right of the
dialog box.
21,845, and pure blue is 43,690. The amount of black is set by the value for
of color in the mix is set by saturation. Pink in the HLS or HSV system would
be obtained by setting hue to red, saturation to some amount less than full, and
brightness to full.
model as if its components were ordered HSL; this puts hue and saturation in
the same relative positions in the data structures of both models.
Color Models in the Dialog Box
The controls in the dialog box are designed for use in the HLS and HSV models:
the user chooses hue by moving the cursor around the color wheel, saturation
by moving the cursor into or out from the center, and brightness (value or
lightness) by using the scroll bar at the right. The way the RGB values vary in
response to the dialog controls is not intuitive, but their responses help to
show how the models relate.
The dialog box cannot exactly match printing's subtractive effect, and it does
routines for converting between RGB and CMY (cyan, magenta, yellow, without
a black component).