Visible only in the presence of light, color, has become so much a part of our lives, our culture, and even our language that we tend to take it for granted. "Until, that is, we start running into problems with its communication and reproduction. It\'s probably safe to say that anyone making a living in a field that involves communicating color information - be they a designer, an artist, a printer, a photographer, or a color scientist - run into plenty of such problems in their course of work. We wind up wrestling with color as RGB, or with color as CMYK, or, if we\'re of a more adventurous bent, with YCC or LAB or some other TLA. What we all too often lose sight of is the simple fact that RGB and CMYK aren\'t colors: they\'re colorant (and often very ambiguous ones) that will, at the end of our wrestling match, produce the sensation of color in the minds of the viewers at which they\'re aimed." -Bruce Fraser (Adobe Magazine)
Light is a form of electromagnetic energy. Our eyes are only perceptive to a small percentage of the electromagnetic spectrum, color, is our response to stimulation by energy at those wavelengths. We can describe light in terms of its wavelength. The unit we use is nanometer (nm), which is one millionth of a meter. Visible light, the only slice of the spectrum that our eyes can see, is that part of the electromagnetic spectrum, shown below, whose wavelengths range from approximately 380 nm to about 750 nm.
The retina is a complex layer, composed largely of nerve cells. The light-sensitive receptor cells lie on the outer surface of the retina in front of a pigmented tissue layer. These cells take the form of rods or cones packed closely together.Light enters the retina on the side containing the optic nerves and travels through the entire layer before reaching the receptors
Colorblind people are those how cannot see the right colors (they have no cone receptors). Such people are called achromats and are very rare.
Other people have rods and one kind of cone. These people are also unable to make any difference between colors and are called monochromats.Color has three fundamental characteristics: hue, brightness, and saturation. These properties are related in scientific terms to the three characteristics of light waves: length (hue), amplitude (brightness), and purity (saturation).
The most important factor of color concerns the hue. Scientifically speaking, the hue is equivalent to wavelength. When we give a color a name, that name (red, light blue, dark green) is one way of organizing the literally millions of hues (wavelengths) of color that exist.
Brightness is a function of the height or amplitude of the wavelength. When we speak of brightness, we are talking about how strong or weak a color is its intensity. Black and white is usually used to brighten or darken an image.
A color is said to be completely saturated or pure if it contains the light of only one wavelength. The more wavelengths that are added the less saturated that color becomes.
Additive Color is the method of creating color by mixing various proportions of two or three distinct stimulus colors of light. These primary colors are commonly red, green, and blue.
The distinguishing features of additive color is that it
deals with the color effects of light rather than with
pigments, dyes, or filters, and that the stimuli come
from separate monochromatic sources.
The most common example of additive color is the color television.
When projected the RGB divides into one color that is white:
The additional colors will divide as followed:
Cyan = Blue + Green
Magenta = Blue + Red
Yellow = Red + Green
SUBTRACTIVE COLOR uses paints, dyes, inks, and natural colorants to create color by absorbing some wavelengths of light and reflecting or transmitting others.
A combination of all three complementary produces black because all colors are subtracted.
Here we have:
Cyan + Magenta = Blue
Cyan + yellow = Green
Magenta + Yellow = Red

The opposite colors on a color wheel are
Called complementary colors:
Green - Red
Blue - Orange
Yellow - violet

And colors that are on the left and
Right of each color is called Analogous
The main difference between RGB and CMYK:
The RGB scheme is used mainly for computer displays, while the CMYK model is used for printed color