There are several color models, also known as color spaces
]). A pixel can be represented in any of them - but not all colors are present in all color spaces. A color space defines components of a color. On a general monitor screen you use RGB (red-green-blue) but for example the color model used in typography is the CMYK (cyan-magenta-yellow-black). But there are more. An image file on your computer is most likely in RGB or CMYK.
There is a color model called HSI (hue-saturation-intensity), you might find useful if you are interested in the intensity part. The conversion from RGB is quite simple, as described here: http://fourier.eng.hmc.edu/e161/lectures/color_processing/node3.html
]. I suggest you consult this article first, but if does not fits your needs, try other color spaces too.
You can play around also with tools like this to get a practical insight on color components: http://www.workwithcolor.com/color-converter-01.htm