Additive and Subtractive Color Mixtures
Two or more colors can be added together to produce a new color composed of the mixture of the initial colors used. This can be demonstrated on a white screen with three colors: blue, green and red. Cyan, magenta, yellow and white are formed from the mixture of these colors (figure 2). One of the requirements in choosing colors to be used in color mixing experiments is that two of them cannot be mixed to produce the third.
Figure 2. Additive color mixtures of blue, green and red to produce cyan, magenta, yellow and white
Subtractive color mixtures involve the selectively absorption of wavelengths. Cyan, magenta and yellow are subtractive primaries. If white light was shone through a yellow filter, the yellow filter will absorb blue and transmit red and green (which makes yellow). Therefore, yellow can be considered as a -B filter. A magenta filter subtracts or absorbs green (-G filter) from white light and cyan subtracts or absorbs red (-R filter) from white light (figure 3).
Figure 3. Subtractive color mixtures of cyan, magenta and yellow to produce blue, green and red
Grassmann's Laws provide a quantitative description of color matching data (Grassman 1855). They hold up well within a prescribed set of conditions (with respect to brightness, adaptation of the observer, size of the field, etc). Grassmann's Laws are useful in quantifying color matching data, but as a rule, color matching data are affected by the following: