Optical color mixing is created through our perception of color. When one looks at two small amounts of different colors laid down side by side the two appear to create a different color. This color is usually something similar to the result when the two are mixed in pigment. The only difference is that when two colors are mixed in pigment, they lose some of their intensity. When two colors mix optically, they retain their intensity and they sometimes appear brighter.
A similar example of this process occurs in color newspaper photos. If you look at a color newspaper photograph using a magnifying glass, you will see not a solid color, but small dots which, when optically mixed, create other colors. Although this process is subtractive and the Impressionists worked additively, the visual effect looks largely the same.