What is the difference between an optical color and an arbitrary color?
Color, scientifically, is the visual stimulation on the color receptors in the human eye from reflected light of a certain wavelength. For example, if you look at a red object, what you are seeing as "red" is the wavelength of the light that is reflected off the object into your eyes. The other wavelengths of light are absorbed and so you do not see them. This is why you can "cancel out" colors with tinted plastic; if you put a red sheet over white paper with blue and red lines on it, you will only be able to see the blue lines, which will look black; the red lines have red light shining on them from the plastic and become the same color as the rest of the paper.
Optical Color refers to a method of coloring with dots of multiple single colors rather than with a mixed color. This allows the brain to meld all the colors together, perceiving them as a single or blended color. This can be seen in comic strips, where the printing method of blending a few choice colors is faster and more accurate than having nozzle tips for every possible shade. This method is also used in LCD screens, where each pixel point on the screen is composed of a RED GREEN BLUE node, which are lightened and dimmed to create a vast spectrum of color. A similar method of art is called Pointillism.
Arbitrary Color refers to the use of color that is "wrong" or incorrect for the subject depicted, but is deliberately chosen by the artist to evoke a certain emotion or feeling. A good example of this is the Uffington White Horse, an enormous piece of landscaped art in Great Britain. The horse is an abstract piece that evokes speed of movement instead of realism, but the striking white of the chalk underneath the green sod give the horse an otherworldly feel, as if it had been inscribed there by aliens. Another example is the work of Andy Warhol, who used powerful primary colors to show soup cans as cultural artifacts instead of simple objects.