Colour modes are used in image editors to specify the basic set of
colour combinations that an image would have. Photoshop supports
the following colour modes:
. RGB
Photoshop’s RGB Colour mode uses the RGB model, assigning an
intensity value to each pixel ranging from 0 (black) to 255 (white)
for each of the RGB (red, green, blue) components in a colour
image. For example, a bright red colour might have an R value of
246, a G value of 20, and a B value of 50. When the values of all
three components are equal, the result is a shade of neutral grey.
When the values of all components are 255, the result is pure
white; when the values are 0, the result is pure black.RGB images use
three colours, or  channels, to reproduce colours on the screen.

The three channels translate to 24 (8 bits x 3 channels) bits of colour information
per pixel.With 24-bit images, up to 16.7 million colours can be reproduced. With 48-bit images (16 bits
per channel), even more colours can be reproduced. In addition
to being the default mode for new Photoshop images, the RGB
model is used by computer monitors to display colours. This
means that when working in colour modes other than RGB, such
as CMYK, Photoshop interpolates the CMYK image to RGB for display
on-screen.
Although RGB is a standard colour model, the exact range of
colours represented can vary, depending on the application or display
device. Photoshop’s RGB Colour mode varies according to the
working space setting that you have specified in the Colour
Settings dialog box.

CMYK Colour Mode
In Photoshop’s CMYK mode, each pixel is assigned a percentage
value for each of the process inks. The lightest (highlight) colours
are assigned small percentages of process ink colours, the darker
(shadow) colours higher percentages. For example, a bright red
might contain 2% cyan, 93% magenta, 90% yellow, and 0% black. In
CMYK images, pure white is generated when all four components
have values of 0%.