Subtractive and Additive Mixtures
Mixing colors in the visual system is different from what you probably learned about mixing colors in elementary school. The visual system mixes lights. When painting, one mixes pigments. The mixture of lights is an additive process -- if you shine a flashlight with a red filter on it at the same location as you shine a flashlight with a green filter on it, the mixture of the red light and the green light will contain all of the wavelengths found in the red light and all of the wavelengths found in the green light. That is, when mixing lights, you are adding together all of the wavelengths found in any of the mixed lights.
When you mix paints, the mixture of the pigments is a subtractive process. Red paint appears red because it absorbs (or subtracts) the shorter (blue) and medium (green) wavelengths and reflects primarily the longer (red) wavelengths. Likewise, green paint appears green because it absorbs (or subtracts) the shorter (blue) and longer (red) wavelengths and reflects primarily the medium (green) wavelengths. When you mix two (or more) pigments together, only the wavelengths that are reflected by all of the pigments are reflected. Thus, mixing red (subtracts short and medium wavelengths) with green (subtracts short and long wavelengths) leaves black (or a shade of gray) -- all of the wavelengths will have been absorbed and none are left to reflect.
Thus, mixing lights will not result in the color mixture rules that you are probabily familiar with -- blue light mixed with yellow light does not yield a green light. Rather it will yield a shade of gray (can you figure out why?)
In the following activity, you can manipulate the proportion of cyan, magenta and yellow in a subtractive mixture. Cyan, magenta, yellow and black (CMYK) are the standard colors used in color printing. Most things that are printed and appear in color were printed with a mixture of those four colors. If you have a color ink jet printer, it likely uses those four colors.
You also can manipulate the proportion of red, green and blue (RGB) in an additive mixture. Most computer monitors, cell phones and TVs use red, green and blue as the standard colors from which all other colors are mixed.
Adjust the CMY sliders to change the proportion of cyan, magenta and yellow in the subtractive mixture. The overlapping areas of the color circles should behave in the way that you learned when you studied mixing paints or pigments. Adjust the RGB sliders to change the proportion of red, green and blue in the additive mixture.
Here are some questions for you to consider. Try to answer them based on what you know about the nature of subtractive and additive mixtures and then either confirm or discomfirm your answer by manipulating the various colors in the activity.