One common application of the RGB color space to display computer graphics on a monitor. For each pixel in the image, independent values for red, green and blue (hence the RGB acronym) are stored. There is a fixed range of red, green and blue intensities displayable by the computer monitor, but by choosing the appropriate combination of red, green and blue light, one can therefore generate any color percievable by humans which happens to be between the [black level]? and [white point]? of the monitor being used to display the image. Typically the values stored vary between zero and 255.
As an example:
* (0, 0, 0) is black * (255, 255, 255) is white * (255, 0, 0) is red * (0, 255, 0) is green * (0, 0, 255) is blue * (255, 255, 0) is yellow * (0, 255, 255) is cyan * (255, 0, 255) is magenta
It is important to note that the intensity of the color output on computer display devices isn't always linear. That is -- even though a value of 127 is very close to halfway between zero and 255, the color output of a computer display device when displaying (127, 127, 127) is measured, you would likely find it to be significantly higher than halfway between the [black level]? and [white point]? of your monitor. This is because most display devices have a [display gamma]?. (i.e. the behavior of most display devices is, unfortunately, not linear in the relationship between color value and output intensity).