Color mode

The color mode of an image specifies which basic set of colors is used to make up an image. You
can specify the color mode in the Image ➪ Mode menu.
Colors within most color modes are stored in color channels. You can view the channels in the
Channels palette.
The color modes available in Photoshop are:
  RGB color mode. Red, Green, and Blue are used to make up the image’s color. In 8-bpc
images, tonal values range from 0 (black) to 255 (white), for each of the three colors.
When the values for all three colors are equal, the resulting color is a neutral color —
black, white, or a shade of gray with no color hue. RGB mode (see Figure 1.9) is used to
display images on-screen because monitors use Red, Green, and Blue light to display
color. It is also commonly used when color-correcting images.

RGB mode channels

CMYK color mode. Cyan, Magenta, Yellow, and Black are used to make up the image’s
color. Tonal values range from 0 percent (no color) to 100 percent (maximum amount of
color) for each of the four colors. In CMYK-colors that are neutral, Magenta and Yellow
have equal values and Cyan has a higher value than either Magenta or Yellow; the amount
of Black determines the lightness of the color.
CMYK mode (see Figure 1.10) is used when printing images with Cyan, Magenta, Yellow,
and Black inks. Typically, RGB mode is used to color-correct images in order to preserve
the maximum amount of colors and to make assigning neutral color values easier (all
three color values are equal in neutral colors in RGB mode), then the image is converted
to CMYK mode. But color correction can also take place in CMYK mode.

 Lab color mode. The Lab color mode (pronounced el, ay, bee, not lab) is based on a standardized
way to describe color that corresponds to the way a person with normal vision
sees it. It is used by color management systems as an intermediate color reference to help
accurately convert colors from one color space to another. It is sometimes used in color
correction and image manipulation. Lab color (see Figure 1.11) uses the following components
to make colors: an L (lightness) component, an a component that contains green
and red, and a b component that contains blue


CMYK mode channels

Lab mode channels

Grayscale mode. Black, white, and shades of gray are used to make up the “colors”
in an image (no hue is present). In 8-bpc images, there can be 256 brightness values.
Grayscale images’ values can be measured in a range from 0 (black) to 255 (white) or
with 0 percent–100 percent of black ink coverage.
Grayscale mode (see Figure 1.12) can be used when preparing images to be printed with
one color of ink. Grayscale images can be assigned a color within most page layout software,
such as InDesign or Quark Xpress.

Grayscale mode channels

Bitmap mode. Only black and white pixels are used. Bitmap mode images are 1-bpc
images (see Figure 1.13). They are often used for simple artwork that lends itself to
reproduction in only black and white pixels, sometimes called line art. Bitmap mode
images can be assigned a color within most page layout software, such as InDesign or
Quark Xpress.


Duotone mode. In Duotone mode, one to four custom inks can be used to make up the
colors in an image. Figure 1.14 shows Duotone-mode channels, options, and curves. A
dutone can be used to add a color tint to a black-ink photo printed in a brochure that
uses only black ink plus one other color. A quadtone with custom inks can give a rich
and uniquely colored look to a photograph. Pantone offers reference books that show
sample duotones printed in various inks and their ink percentages.


Bitmap mode channel

Duotone mode channels, Duotone options, and Duotone curves

The Duotone-mode images can be referred to as monotones, duotones, tritones, or quadtones.
Their values are typically measured with 0%–100% of ink coverage. Duotone mode
is used when two, three, or four custom ink colors (other than the typical CMYK combination)
are used in printing.
Duotone mode separates the overall image into multiple custom colors, and the Duotone
mode curves can be used to adjust the percentage of ink in specific areas of the tonal
range of each color.
n Indexed color mode. Up to 256 colors can be used to make up the colors in an image.
Colors are stored in a color lookup table (CLUT). Indexed color images are typically used
for GIF format images destined for on-screen display (see Figure 1.15). When you save a
GIF by choosing File ➪ Save for Web & Devices, the image is automatically converted to
Index color mode.
You can generally or selectively reduce the number of colors in an indexed color image to
reduce the image’s file size, which decreases download time for Web images. In an effort
to improve an image’s appearance while at the same time using fewer colors, you can also
take advantage of dithering, the simulation of a third color by generating the appearance
of two colors being mixed together.
FIGURE

Multichannel mode. Multichannel mode uses only spot channels (see Figure 1.16), channels
that store parts of an image that you want to print in custom-selected inks (sometimes
called spot colors), such as inks from the Pantone color library. In Multichannel mode,
you can put certain parts of an image on each spot channel and assign custom-selected
inks to the spot channel.
For example, you can put a custom-red-ink logo on the spot channel for red ink and a
custom-blue-ink background on the spot channel for blue ink. You can also put image
areas that you want to print in clear varnishes on spot channels.
Spot channels result in inks overprinting each other, so if the logo mentioned previously
is to print in red ink only, then the area corresponding to the logo in the blue spot channel
must contain no pixels in order to avoid blue ink printing on the red ink logo.

Multichannel mode spot color channels in a two-color image

(Sourse. photoshop bible)