Blue and yellow make green - don't they? Maybe if you are mixing paints, but if you look carefully at your computer
screen with a magnifying glass you will see that its display is made up of very small red, green and blue rectangles.
Obviously it displays green by simply using the green rectangles by themselves. But how does a computer screen display
yellow? The truth is that mixing paints to achieve a specific colour is a subtractive process whereas the computer screen
achieves specific colours by an additive process. Not surprisingly, just like mixing paints, colour printers also achieve
specific colours by a subtractive process using magenta, cyan and yellow coloured inks.
Above is colour mixing program. In the top left corner of the colour mixing program is a square of computer screen that is
initially white. Beneath this square is the current state of the small red, green and blue rectangles within the square of
computer screen. When the square of computer screen is white, the small red, green and blue rectangles are all operating at
100% brightness. To the right are the amounts of magenta, cyan and yellow paints or inks that would be required to achieve the
colour displayed by the square of computer screen. In the world of paints and inks we usually start with a white sheet of
paper so no paint or ink is required for white.
You may change the amounts of magenta, cyan and yellow to observe what happens
to the square of computer screen. Notice that increasing any one of these actually reduces the brightness of one of the computer
screen colours. Thus you see that mixing paints or inks to create a colour is a subtractive process. Notice also that when
magenta, cyan and yellow are all 100% you have totally "blacked out" the white sheet of paper. In computer screen terms
black is when the small red, green and blue rectangles are all off.