FAQ - why are the primary colours on the computer monitor Red, Green and Blue? I thought they were red, yellow and blue.
Yes - mixing of pigments does indeed use red, blue and yellow - well actually in printed images, it's usually magenta (red-blue) cyan (sky blue) and yellow, but there is much room for flexibility there in the choice for the primary colours of pigments. Red yellow and blue were in fact sometimes used in the very early days of colour photography and printing, and they are still considered primary colours for artists paints.
The reason colour monitors use red green and blue rather than red yellow and blue is because they mix light rather than pigment, which leads to a different set of primary colours. Again there is much room for flexibility here too, with other possibilities available. For instance violet would be a possible choice instead of blue. The colour sensitive cones in our eyes have response curves with fairly broad peaks, and each one is sensitive to some degree to light throughout the colour spectrum. They don't even peak at red green and blue particularly. So there is much room for choice for the primary colours of light - and for pigments. The system of red, green and blue light is one that happens to work well for for rendering natural scenes and colour photographs.
The primary colours of light and pigments have been standardised to six colours - red, green and blue for light, and magenta, yellow and cyan for pigments. This standard makes it possible for colour monitors, cameras, printers other equipment to work together harmoniously to show colour pictures. Indeed, when you print out a picture from the monitor, its red green and blue values get converted into yellow, cyan and magenta pigments (usually with black as well) for the printer, so the printer is using a different system of primary colours from the monitor, but the standard ensures that you see the same colours on the printer (at least, very close to the same anyway) as you see on the monitor.
Oneeasy way to get a first idea of how it all works is to think of sunlight as made up of a mixture of red, green and blue light, as you might make its colour on a computer monitor. The situation is much more complex as sunlight really consists of a continuous spectrum of light through all the shades of the rainbow, but this will get one started thinking about how it works.
Then, green on a printed picture is a mixture of yellow and cyan (sky blue) pigment, which isn't so much of a surprise as that is pretty much how one learnt it at school. But nevertheless, how exactly does that work?
The yellow pigment reflects a mixture of red and green light, so it removes blue from the incident white light. The cyan pigment (sky blue) reflects a mixture of blue and green, so removes red. When you mix those two pigments together you remove both blue and red light from the incident light, leaving only green, so the result looks green. So that is how it works.
It isn't so hard to see that the green ink on your computer print outs is often a mixture of two colours - but its blue ink also may not be a pure pigment, rather a mixture of magenta (a kind of purply pink) and cyan (a sky blue), which may come as more of a surprise. This time the magenta pigment removes green from incident light, and cyan removes red, so when you mix those two pigments together you are left with only blue out of the original red + green + blue.
The actual colours you can show on a monitor are limited actually, because they have to be made by mixing only three colours of light rather than a continuous spectrum. You can show all possible hues, all colours of the spectrum and also the purples between red and blue on the colour wheel, so it is pretty good, it gets all the hues. But sometimes the results are a little unsaturated - washed out in colour. The really pure spectral colours are the ones that are hardest to get, and a computer monitor never quite achieves them. When you set the saturation to 100 percent, this just sets it to the most saturated version of that colour for your computer monitor.
Indeed, most spectral colours are shown slightly unsaturated on a monitor. The yellow on a monitor may look a pure spectral yellow to most of us - but actually it has a bit of white mixed in compared with pure spectral yellow that you find in a rainbow. You would get a purer yellow in in a monitor if it used six primary colours of light instead of three. One could work with more than three primary colours of light - maybe with six colours - but this sort of thing is normally only done in specialist equipment for colour researchers. So if you show a colour disk on a computer monitor, there are other colours outside the rim which can't be shown, and they are more a bit more saturated and pure, particularly in the intermediate positions like yellow, cyan and magenta or violet. One reason we don't notice it so much is probably because the choice of primary colours for the monitor was chosen to render natural scenes and colour photographs particularly faithfully. Bright pure saturated yellows and pinks and violets aren't so common in nature.
Also white is always the brightest colour on the monitor and none of the other colours can be shown quite as bright as the white you get with all three of the primary colours of light shining simultaneously.
The reason for the use of red green and blue lights as primary colours for light stems from the fact that our eyes have receptors sensitive to three colours, though the actual colours that our eyes are sensitive to aren't quite these ones exactly in fact. Here is a link that also goes into many technical details of how colours are shown on monitors and relationship of those to human perception: Colors and Colorimetry . Another interesting link is this one on Color Models , see also Computer Graphics Color models . The model for the colour disk here is the HSV (hue, saturation and value (brightness)) colour space which can be shown as a cone, hex-cone or cylinder- here is an on-line link to a page with a cut-away drawing of it as a single-ended cone: What is HSV .
For a short overview of Maxwell's colour triangle and the idealised red green and blue, see The Composition of Color .